Should I stay or should I go?

In recent days, I've become a bit unsure about what to do next year when my current contract runs out. Should I stay? Should I transfer to the London office? Should I never sign the contract to return and resign?

The reason I felt a bit depressed in my job in recent months was that I had the feeling many good people were leaving, and that this trend was stronger than ever. In conversation with others in recent days, I've realized that it might be a complete misinterpretation. On average, people stay in consulting for about 2 to 3 years. That's exactly the stage I am in now. So it is just logical that I feel more people than ever are leaving, because now is the time when the people I know best and feel closest to are starting to leave. Probably, the same amount of people (or even more) of my current tenure left when I joined, but I didn't know them so I didn't care about them. Maybe it's just a question of getting used to the high turnover (20%+)?

I guess this is a general problem about making decisions in your life when you're unhappy somewhere: is the problem just your attitude and how you deal with the situation? Or would escaping the situation make things better? To what extent can you influence how happy you are somewhere (in a job, in a certain city, in a relationship etc.)? And when is the time to move on? I guess the logical answer is "try to change it/yourself and when it still doesn't work, move on". But I'm not sure I've done enough actively to make things better. I've done my best to pick projects with nice and likeable people and reasonable workload, but the results have been mixed.

So I could talk about hope. Am I optimistic that the next project things are going to be better and I end up working with nice people in a nice environment? Or will I always be disappointed because I'm too picky and am going to moan about anything? Thinking of the times I was happy in my job (and there were times that I was), I think it is true that I'm a bit picky, but still I know I can be very happy and motivated if I'm with nice people, doing interesting things and getting some sleep. That's all I'm asking for.

One option I'm considering is, instead of leaving my job, transferring to the London office. The advantages would be a more diverse workforce, less travel, and less workload, while still having all the same advantages the job offers. I could better assess if with a bit more sleep and a bit less travel, I'll end up liking what I do again.