
tl;dr: insecurities caused by a melting pot of horrible people/situations - his grandfather, ash's friends, omegaverse biology, etc.
i think in lyle's position, it's because ash is his first love & everything after having been confined & leashed by his grandfather. to lyle, ash is the most special & important person to him - but finding out that ash had a person j as important & special before is probably a huge point of insecurity for him. it doesn't help that the way ash's friends are speaking of ash & his ex's relationship as if they're all shocked he got together with lyle when he & hazel should've been end game.
+ lyle j has many other issues caused by his grandfather. he already feels inadequate as an alpha, while lyle's ex was an omega, so biologically, lyle's ex was a "better" partner because they're capable of bonding (not sure if you're used to omegaverse, but bonding is quite a heavy thing, either physically or emotionally, because it's akin to dependency). lyle also doesn't feel like he's attractive in any sense - cute, adorable, pretty, etc. lyle doesn't believe he's any of these things, while omegas are more often than not always all of these things.
+ another point of insecurity for lyle is probably because he feels like he loves ash more than ash loves/will ever love him because ash might have loved his ex more. lyle has wanted to marry ash for a long while, only he's never raised it because aiden told him it was too soon. then he finds out ash had wanted to wed someone else (isn't what his friend said, but insecurity always messes with things) while never raising with him.
I’m really confused. If you’re dating someone now, why does it matter who they dated in the past? Why does it bother Lyle in this case? Genuinely asking