I know this has been stated before but I have Thoughts™.
Lance has stated a couple times that he really looks up to Shiro and that he’s his hero. Lance tries to impress Shiro, tries to get Shiro to give him a chance, but Shiro, bless him, consistently brushes Lance off. It’s not malicious, it’s just dismissive, which might be worse. If you can’t get more than just apathy from someone who affects you so much, it’s frustrating.
Lance is seen as comic relief, despite establishing himself as a good strategist and loyal team member. Yeah, he’s a goof, but he’s also deeply insecure, using jokes and flirting to hide his doubts (relatable tbh). Always taking a backseat for Shiro’s attention is already grating on Lance. Hardly being listened to by your teammate and leader isn’t conducive to good self-esteem, and it’s showing in how Lance doubts his own benefit to Voltron.