Why are people so fake?

Ok so my friend just randomly started talking to this guy that used to be our friend until he decided to "break up" with us. And just yesterday she was talking about how much she didn't like him and this and that. And I never really had any problems with him so if anything I wouldve excepted me to talk to him first but it's been months so it wouldve been awkward. I really don't understand how she can talk bad about him one day and talk to his face like they're friends again. And he asked her if they were cool and she said yeah. So I'm like wow. So why do you think she would do this? Also her friend is really fake to because she talks about all her friends behind their back. And then my friend talks about her to me and says other things about her too so I'm like wow. And then she was saying she was done with her but the next day they were holding hands and this happens a lot so I just get used to it. It really makes me wonder who I can trust cause it's like are going to repeat what I say to someone else like you talk about what your friends say. I just wanted to know your thoughts on this. Thanks!