Private conversation I shouldn't have seen

So my boyfriend and I broke up over the weekend... We got into an argument I broke up with him Saturday.. But we made up that same day I decided to give him another chance. The next day I brought it up again and he ended up breaking up with me.. We're back together now and super happy. Buuutttt I saw some messages from his brother were my boyfriend reached out to him to tell him about the break up.. He said he did it. "Because it's the right thing to do, I'm gonna end up cheating on her and I don't want to put her through that" and the brother tells him "it's ok you didn't even like her at the beginning and u never did" ... I really wish I hadn't seen this .. Our fight was over trust issues .. Now I'm worried that he will cheat? But why would he get back with me ? This is so confusing