-Sharing rooms is usually cheaper than getting a single room, and sometimes even cheaper than getting a single bed in a dorm.
-You can go places you wouldn't feel safe or comfortable going alone.
-You can pool together funds for experiences you couldn't otherwise afford solo (renting a car for example).
-Someone you trust can taste that weird local delicacy made out of insect larvae and tell you whether it's any good before you try a bite (Stephen, I'm looking at you and your Korean silk worms).
A good travel buddy can let you go further and have more fun. And they will probably get you to try things you would never have tried on your own, many of which you will really enjoy. Some you won't, but that's the risk you take.
A bad travel buddy can make both of your trips an absolute nightmare. So you want to choose wisely. It's like having a roommate who you see almost every hour of every day for the duration of your trip. Even some of your best friends, and yes, your significant other, can be terrible travel companions.
Here are a few things you need to discuss before deciding to hit the road together, with a few extra notes for couples at the end:
-Budget. This is probably the main cause of friction between travel buddies that aren't romantically involved. If your budgets don't match, every time one of you spends money, one of you will feel like cheapskate and pressured into buying things you can't afford, and the other will feel like an extravagant showoff and like they're being forced into second rate... well, everything. Compare notes on how much you want to spend on food, transport, and lodging, and what your idea of a reasonable price for a day's activity is.
-Travel Speed. Some people like traveling slowly. Some people don't. This also ties into budget-- faster is more expensive. But if you feel rushed or bogged down by your companion, you're going to start to resent them. NOTE: The more people you travel with, the slower you usually travel, just by virtue of everyone having to see if everyone else is ready to do anything.
-Conflict Resolution Skills. You could both be saints, but at some point, I can almost promise you, something you do is going to get on the other person's nerves, and vice versa. When you two have differences, can you resolve them in a mature, efficient manner that leaves both of you feeling okay about it later? This should be discussed, and possibly tested before you hit the road.
-Interests. Having different interests is fine-- the problems start when your buddy is actively disinclined to try something you want to do. For example, if you're an avid rock climber, and your buddy has a panic attack if he's outdoors for more than four hours at a time, you're gonna have issues. Likewise, if your goal is to try every different kind of beer made in a region of the world, and your buddy hates bars, trouble is brewing (sorry). Having different interests usually ends up opening new doors for people, showing them things they wouldn't ordinarily have experienced. But if people prevent you from pursuing the interests you hit the road to pursue, it's going to be a problem.
-Cultural Respect. It's embarrassing to travel with someone who continually puts their foot in their mouth. We all do it once in a while, but some travelers seem to do it every five minutes. If you are going somewhere with a different culture, make sure you're going with someone who will treat it with respect. They don't have to know all the little rules for being polite, they just have to be thoughtful about figuring out what they are and doing their best to abide by them. Having to apologize for your friend's single mistake is a good story. Having to apologize for your friend's repeated mistakes over and over is just aggravating. And I can tell you firsthand, the fifth time you hear "Well I'm just not used to having to _____, because we don't do that back home," you're going to want to smack someone.
-Physical Condition. If you want to go walking all over town every day, and your buddy has to stop for breath after a flight of stairs, you might not want to travel together. Yes, the out of shape one will get in better shape, but it's going to be a long and arduous wait for both of you before that happens. It's hard to predict how much you'll be walking around, but you can guess what your tolerance is, and choose someone with a similar level, so you can walk as much as you want, and don't feel bad calling a cab when you don't want to walk anymore.
-Special Needs. Diabetic? Vegan? Pack-a-day smoker? Your potential traveling buddies need to know before you leave, because it will affect their travel experience, too.
-Alcohol/Drugs. Similarly, if you intend to get hammered and stoned every couple of days, you need to make sure its on your buddies' agendas and budgets as well, since they're likely going to be the ones that have to take care of you when it happens. It's easiest to just travel with someone who wants to try what you want to try when you want to try it, because then at least you can take turns being the designated responsible guy who knows where the hostel is and can walk straight.
Now I would like to include a couple special notes for a special brand of traveling companion: those in relationships. You could be married, you could have just met in a hostel last week and really liked each other. Either way, you need to be clear on a couple (ha) minor points:
-Make an effort to know the cultural norms of dating and public displays of affection. Some places have couples making out on every corner. Others are scandalized by hand holding. Still others find it offensive to even see a woman traveling with someone who is not their husband or relative. For these last places, choose one of those two stories and stick to it. This is not an opportunity to try to force your cultural norms on another nation, even if they seem more progressive.
-If you're sexually active, think about when you will and won't be able to get private accommodations, and bring plenty of whatever birth control methods you use with you. Local variants are not always reliable.
-This is a tough one to approach, but if you are planning on traveling together for more than a month or so, and have never traveled together before, it might be worth discussing the worst case scenario: a mid-trip breakup. Even if you can't imagine it ever happening, being together 24/7 in completely different surroundings and occasional discomfort can bring out sides to people you never knew existed. Is your plan and schedule flexible enough to allow you to go your separate ways?
-On that note, even if you are really happy, you should plan some time apart. You'll each get to do your own thing that the other person doesn't like, whatever it is. And after that, it makes meeting up and being together again feel that much better.
And there you have it! Hope this help you find a traveling companion or three that makes your trips more enjoyable and memorable.