Facing The Reality
It didn't take long for Tim to realize that he and Cassie weren't going to work. It wasn't their years-long friendship, or their innate differences, or even the (too) many and varied ways in which they were the same.
It was the simple fact that two broken people just couldn't fix each other by being together.
And unfortunately, since Cassie was really his only friend left – aside from any of the other Bats – when they broke up, he didn't have anyone to talk to. He and Cassie might not have been working well, but he'd felt a lot less lonely when he was with her, even if he really wasn't. He couldn't open up to her, nor she to him, and they'd been mostly going through the motions of trying to find comfort in each other.
Human emotions being what they were, they'd done a pretty good job of pretending and faking their own depression out.
Martian emotions were different, however, and when M'gann approached him with her quiet offer of friendship, Tim didn't let Robin say no. Tim appreciated her company a great deal; while teaching her about life on Earth, she in turn told him about her life on Mars. M'gann had an empathy that Tim was drawn to, so unlike his own brooding reticence. It drew him in as no pleading entreaties from Cassie ever could, no matter their own shared history.
When M'gann offered him more than friendship, Tim was scared, but Robin wouldn't let him back down from what, and who, he wanted. The feelings he had for the White Martian wouldn't be denied.
And the reality was, he and M'gann were a good match. In the long run, maybe they wouldn't last, but the only way for him to find out, was to be with her.
And both Tim and Robin were determined to sate their curiosity on that matter.