Sometimes it's just that the ml is just absolute shit, so the interaction with the so called second ML feels better. It's like when stories suddenly make the mc feel doki doki out of thin air for the ml so it's no chance for second ml, who then for some reason helps them get together instead being absolutely NOT heartbroken.
Or sometimes it's just the circumstance, best example of this is 'a step mothers marchen', it just does the second ml the best but in that case the ml is amazing too so it's just angst
I don't have that feeling in every story either. But some characters just stay with you. And subconsciously you start to root for them. I don't mind if they end up together but it pisses me off if they torture him till the end for nothing. So in those cases I don't feel happy at all even with the happy overall ending.
The only reasons I feel second male lead syndrome is cuz a LOTTT of the male leads r just douchebags, OR(tho this is rarer since I mostly just hope they find their love one) has a rlly sad background that I pity them.
I never experience second male lead syndrome and I'm feeling left out... how can you root for the other guy who, despite having so many chances never had the balls to confess until after someone else suddenly swoops in all direct.