In the end running is a solo sport, and that by its nature attracts some "characters," obsessives, and the just badly socialized. Luckily, most of us aren't like that. The way that everyone can hurt in the same way at different speeds, means that running has a better sense of community than a lot of sports. There's few other sports where Olympians say "hi" to hacks as they pass each other on runs. There's a lot to love about the social aspects of our sport.
Quite apart from that, training with others involves balancing the needs of each person in the group. If someone's needs are not being met it's easy for resentment to build up. People can also say some dumb things when they're tired and light-headed.
But running is part of life, not distinct from it, and relationships will break as they do in other areas. If you've been honest and friendly and true, and the friendship still breaks up there's not much you can do, except move on after being sad for a little while. Say "hi" if you see them because you're a good person, not to spite them.
Good training partners are a valuable thing.