hints broadly as to why that is. Let's take a good long look at the visual imagery to see what's going on.
As we see in Panel 1, we have Janet smiling smugly at Martha because she has successfully managed to take Michael from her while Martha frowns at her as if to say "We aren't friends any more, boyfriend thief!" We also notice that Mike and Lawrence have no real idea what's going on because they ain't looking at the girls' faces.
This switches to Panel 2 where Mike glowers at Lawrence who smiles and does that stupid palms-out gesture the characters do. The message he's trying to convey is that it's not his fault this happened, that Martha threw herself at him. Again, the other dance partners have no idea what's going on.
We find out why it is that Mike ends up telling Gordon pure twonk about how Mart got bored with him and went with someone else this September when she looks at him with a despairing frown when he looks at her as if she's something he dug out of his ear or something. This tells me that at some point, someone came along and told him that girls get bored with boys all the time and the stupid piece of shit believed him because why would his friend Lawrence lie about that despite throwing him under the bus all those other times?
Yeah. I went there. I got the T-shirt and brought back a bunch of souvenirs. The reason that Mike believed bullshit about Martha for all that time is that someone who he never realized doesn't actually like him and views him as someone he can torment with impunity because he's gullible and pathetic told him a lie and the pea-brain is desperate and stupid enough to believe everything he says no matter how often he gets in shit because of it.