Why would a guy behave like this if he isn't serious?

For example, going on dates with you without having sex at the end of the date, paying for everything (foods and drinks), being attentive towards you (looking constantly at your face to see if you're feeling comfortable) and doing little sacrifices to make you feel happy (let you choose the place you want to sit in, what you want to eat, what you prefer in sex...). At the same time, texting you constantly, talking about a lot of things (not always sex) and sharing his problems with you. Also, inviting you to go for some drinks with his friends. Or feeling jealous if you mention another guy. Showing photos of his childhood. One day I even noticed that he prolonged his way back home only to spend more time with me.
But, at the same time, showing things that you both are not serious. For example, writing you to invite you to come over late at night, remembering you when he's drunk, or disappear on you for a month and then reappear as if nothing had happened.
It's a guy I've known for more than one year and this has been pretty much our relationship. When we met he had a girlfriend but they split very soon afterwards, he then started this type of romance with me without being anything clear. And even during this time in which he's been pulling away and reappearing, I was trying to create a more serious love life by dating other guys (and during these times I stopped answering him), but we always end up getting in touch again.
What's going on here?
Why would a guy behave like this if he isn't serious?
Add Opinion