Here's my take on it.
He didn't want to be alone when things got weird with the new woman, and he didn't have anyone else on the string.
He called you and you hopped back on board.
He has a need to have new partners. A lot of people do. Being alone or being with one person just doesn't give that addiction "hit" - that intense dopamine/oxytocin rush. Many people jump from one new sexual partner to another, or have several at once.
It's not you (or, it's not all you.) He's doing what he's always done.
Scorpion and the frog:
A scorpion wants to cross a river but cannot swim, so it asks a frog to carry it across. The frog hesitates, afraid that the scorpion might sting it, but the scorpion promises not to, pointing out that it would drown if it killed the frog in the middle of the river. The frog considers this argument sensible and agrees to transport the scorpion. Midway across the river, the scorpion stings the frog anyway, dooming them both. The dying frog asks the scorpion why it stung despite knowing the consequence, to which the scorpion replies: "I am sorry, but I couldn't resist the urge. It's in my nature."