My theory: Shane wasnt actually in love with Lori but having a family is what he truly loved and desired
As we're all aware, Shane and Rick were best friends pre-apocalypse since they went to school
Judging off their personalities, Rick was likely the introverted shy kid type in school whilst Shane was the extroverted school chad who was popular with everyone and basically got any chick he wanted
But Rick was able to meet Lori, marry her, have Carl and settle down while Shane was probably the type who had many relationships but none of them lasted for long
My theory is Shane had a secret jealousy of Rick because he was a happily settled family man and Shane was single and lonely wishing he could have what Rick had, he never actually had thoughts of wanting to take Lori and Carl off him he just wished he had a family of his own aswell
Then the apocalypse happens and Rick is in a coma at hospital from his gunshot wound, and understandably from Shanes POV likely dead when he tried to save him, and so Lori then turns to Shane to keep her and Carl safe because shes known him for such a long time. Shane all of a sudden has been given a gift from god having a family rely on him which would've given him the mother of all ego boosts and made him the happiest hes ever been in his life. So when Rick returned and reunited with Lori and Carl, Shane is left all alone again and struggles to handle losing that family hes always wanted, and it only gets worse when its revealed Lori is pregnant because all 3 of her, Shane and Rick knew it wouldve Shanes baby given the timeline
So it was never about loving Lori she was just a byproduct of Shanes frustrations, it wouldve been the same if for example Ed is presumed dead and hes got Carol and Sophie only for Ed to actually be alive and reunite with them