Is it possible she threw him for a loop unintentionally years ago and he spent a lot of time getting past it...and he feels toyed with? Is it possible he feels it's a bit early to be treating things so seriously?
If it were me, I'd explain myself and leave him be. He'll come around.
In response to the question about guys being insecure, the ones in particular that are, have suffered at the hands of being hurt badly and are truly confused about what to expect in a relationship. They spend a long time learning to put their guard up and not worry about caring, they're very protective of who they let in, and it'd serve a girl right not to toy with that guard unless she intends to uphold the place she wants to capture.
To many girls, just like too many guys, are too fickle with their interests. They make promises they can't keep, and then toss them when they no longer fulfill her void. Because of that, smart men know to give a waiting period before accepting niceness as truth.
And then some of us, like myself, just treat niceness as friendliness so we don't have to think about things without direct clarification otherwise.
I don't complicate it so I don't confuse mySELF and spend my days in what ifs or making more out of things I don't know to exist and don't care to entertain if I don't think they do.
There are many reasons it could be. Some people just prefer things with clarity.