I believe it is a feeling and a decision as well. My ex got caught up with the "feeling" with is affair partner and determined that he must not love me because he hadn't had "that' feeling for quite a while.
I told him you couldn't sustain that feeling of "new love"...your body would give out. He thought I was lying. Actually I think it was the other way around he lied to justify his leaving.
But anyway, you choose to stay because of commitment, family and a mature love that has developed and created a bond. The first love feeling passes and with it many an affair(unless of course they rush off and get married and then you get another divorce!)
Just my take.
kat
Me-53(and learning!) S24, S21, D18, D17 Just keep swimming, Just keep swimming. Dory