Why does an ex return?

So me and my ex were just friends and texted each other for 3 months and somehow got in to a relationship. We did not get physical or anything and during our first month i caught him texting another girl basically flirting. Which according to me is cheating.
And so i contacted this girl and told the girl that i am his girlfriend and asked her to ask him if he was taken? And he told her that he was single. So that statement coming from him was self explanatory on what he sees me as.
So i told him i know what he’s doing and didn’t hear a word from him and blocked him from everywhere. I was devastated honestly because it was my first serious relationship but I didn’t wanna listen to my emotions so I did what I thought was right. All i wanted was an apology which I didn’t get. However I’m not a grudge holder and get over things easily and so i unblocked him and moved on with my life. Fast forward to now 7 months later he sends me a req on Instagram? What should that mean?
I’m new to these stuffs, help me out
Why does an ex return?
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