FSP has a post up today about the importance of liking your grad students. I don't have grad students I advise, but I have had long-term working relationships with the people I've worked with on field projects.
When you're trapped with someone for 10, 12 hours a day, perhaps in the middle of nowhere, or in extremely stressful situations, personalities can clash. Or maybe you find yourself driving 7 hours to a field site with someone you don't know and that you have nothing in common with (and you can only talk about work for so long).
I've worked with guys who continually made passes at me (and we were the only people for miles, ugh), drunks, miscellaneous small supply kleptomaniacs (hmm...where did all my screwdrivers go?), and folks with hair-trigger tempers. I didn't like any of those people. They were aggravating in a thousand different ways.
Depending on schedule and the project requirements, I have some ability to assemble my own field team. The selection process is based on what the project needs and the ability of the available personnel. I may not like those people personally. But I'm a grownup, and I am a professional, and I can deal with people I don't particularly like in order to get the needed work accomplished.