how did you decide to move in together?

When and why did you and SO decide to move in together? Is it weird that I've been with my SO 2 and a half years and he is still not seriously considering moving in together? I stay at his place around 3 to 4 nights a week. The lease is about to be up on his place and he is looking at places to move to. When I mention the possibility of us looking for a place for us he says he will think about or we will talk more about it or makes a joke about what would be different and that's it. We're both adults. We would be fine financially. We're serious in our relationship. So I don't get it.