He came in to the Spot for the first time ever towards the end of s7, saw how gorgeous the owner was and decided to make that his regular moving forward.
Throughout all of the time between season 7 and season 8 (while Dean's in Purgatory) he tries to "be there" for Sarah as she and the others work on bringing Dean back. Flirting that goes completely unnoticed by Sarah because she's still with Dean, not physically at the moment but in every other way that counts. Lucas gives up.
... Until season 9. She's single now so he makes his move and she bites. "Friends with Benefits only"- Sarah from day one.
She sticks to that promise too so they barely see each other except for a super occasional hook-up. Sarah never spends the night and she never brings him to the compound where she is most of the time, nor to her apartment. Ever.
They sleep together one more time at the end of s9 then she vanishes for weeks going on months. Next thing he hears is Dean's got the Mark of Cain and Sarah's focusing on helping him. Lucas gets a little more jealous.
Phillip at one point full-on says "I don't like you drooling over my sister". A physical fight breaks out that, luckily for Lucas, Phillip hasn't brought up to this day.
By the time season 11 comes around Lucas and Sarah have spent all of one month together- if that- collectively yet he's still convinced he's in love with her.
He finally TELLS HER how he feels and it goes about as well as you'd expect. They end the conversation on a friendly note, surprisingly, and agree to go their separate ways.
... Until word gets out about a demon on the loose somewhere nearby and Lucas decides he wants Sarah to be his hunting partner for it.
It takes a lot of convincing but he finally gets her to agree- all while thinking in his head this might get her to feel something for him, he's gonna save the day! He's gonna be a hero!
Sarah agrees to be the bait, she puts her anti-possession bracelet in her pocket and cuts at the tattoo on her hip. "You're gonna get to me in time, right?" "I promise."
The demon comes because they recognize Sarah as an important person in the Winchesters' lives. They possess her ... and Lucas does absolutely nothing. He'd somehow forgotten how fucking terrifying demons were and gets scared. He runs off while the demon's torturing Sarah physically and emotionally, emotionally because they let her see Lucas running off-- and physically because they're cutting her up. Wanna get back at the Winchesters you gotta strike hard, after all.
Luckily Sarah had told Phillip what was going on and why she'd be gone for a bit that week and he didn't like that so he followed her just incase while keeping some heavy distance between himself and the pair so they wouldn't see. He got to her right before the demon could take any fatal blows, it left her lying there on the ground, Phillip put her bracelet back on her wrist first, picked her up as gently as he could and called Sam for help. They got her back to the Bunker in time.
Sarah hasn't spoken to Lucas politely since. He's permanently banned from the restaurant.