What are some of the things you've learned about traveling successfully with friends? Here are some of my thoughts (and why I tend to travel solo):
If you can't agree on where to have dinner in your home city, seriously reconsider spending weeks negotiating how to spend your time.
After a certain age, it is more efficient to have separate rooms than to save a few euro by sharing. Every person has their own meaningful (to them) rituals regarding bed and bath, so why look for trouble?
3a. Listen carefully during the trip planning stages. If everything is "I leave that up to you", is this code for "I expect you to take care of everything, including helping with my luggage and waking me in time to get off the train"?
3b. Listen carefully to the lack of participation by your friend(s) in the planning stages. Do they really want to go on your vacation, or will their private agendas become public once you clear immigration and you have invested time and money planning what you thought was a perfect trip for all?
4a. Are you really comfortable being the chief planner?
4b. Be prepared to make spend significant amounts of time making amends for things you took for granted or that your friend(s) didn't mention were important to them.