Things were bad, now they're good. Why did he change?

We've been living together for 15 months. It was a hard year, he was distant, unemotional, unaffectionate. I suspected him of cheating and it almost blew us apart. Now he's attentive, sweet, loving, I don't get it. I wish I could enjoy it but it's keeping me up at night. I want to ask him but I don't want to look at gift horse in the mouth. You know, push my luck. I have the inescapable feeling he's been messing around for months and is "now" ready to settle down. It's almost disturbing how much he's changed. I feel like, why wasn't it like this the whole time?? What was I being tested? And I ped?