If you ask Sirius, it was all a big coincidence. He didn't mean to get back with you. You've been broken up for a while, and it was mutual; you outgrew each other. Or something close to it.

Maybe he persuaded all of your friends that it was actually a good idea — letting him back into your life. He swore to Marlene, Mary, Remus, and Lily that he changed and he's better. Like seven percent better but still better!

Now he was on your doorstep, acting like he came here by accident. And those flowers in his hand? Just stumbled upon them, seriously! Room of Requirement, you know?

"So, um, can I come in? Elves gave me these chocolate biscuits. I totally forgot that they were your favorites. Crazy, huh?" He smiled charmingly.