It deepens your bond.
It'll give you stories to tell -- and lean on -- for years.
Travel stories are a great way to break the ice when the two of you meet new people, and they're guaranteed to generate some genuine laughs. But more importantly, these shared stories will be things that the two of you can reminisce on forever. And when you don't live near enough to have many common experiences from recent years, shared stories become really, really important.
It shows you things you never knew about him or her, even after you thought you knew it all.
You never knew your friend liked cow tongue, because you've never seen her around cow tongue before. You never knew your best friend practiced meditation, because you've never seen him in a temple before. You never knew your best friend had a thing for electro-pop raves, because... you get the picture.
It teaches you what your friend likes and doesn't like.
Maybe she likes to have philosophical talks on planes, and maybe he doesn't like when you steal bites of his food. Either way, you'll learn new intricacies that help you understand your best friend is in an even deeper way.
"It we got through that together..."
The time you missed your flight and had to sleep on the floor. The time you got food poisoning and had to hunt for a doctor. The time you had a hostel roommate who snored like a thousand cows. Once you've conquered a travel disaster with your best friend, you know you can make it through ANYTHING together.
It gives you something to look forward to.
Research shows that most of the time, planning a trip brings more happiness than taking the trip itself. And happiness is that much sweeter when shared with your best friend.
You'll have someone to share every view, meal and laugh-out-loud moment with.
Solo travel is great, but sometimes you just want your best friend to see things, too.
Sleeping in the same hostel bunk won't be awkward.
Nothing's worse than the obscure silhouette of a stranger sleeping above your head in a hostel bunk. And with friends you don't know as well, sharing a hotel bed feels just a little weird. Hooray for best friends!
You'll have someone to stand up for you when things get scary.
It's hard to stand up for yourself during an argument with an airline agent, or when the hotel room is priced higher than advertised. A best friend will make sure you don't get walked all over.
More skills = more possibilities.
You speak Spanish, and he speaks German. You know how to find the best restaurants, and she's a connoisseur of rock-climbing tours. More interests, skills and tools in your arsenal means that you can do more activities together on your trip.
Someone will know what you've been through, even the things you can't describe.
You know when the sunset is perfect, but you just can't capture it on a camera? Your best friend will remember it with you.
You have a constant wingman for all sorts of relationships.
Not only will your best friend accompany your romantic explorations at the bar, but they'll make it less awkward to chat up the taxi driver or get to know the host at your hostel.
Someone will be there to hold your airplane barf bag.
A sign of true love.
And train and bus rides will be WAY more fun.
Think of the sudoku, crossword and M*A*S*H possibilities!