The first time they kissed, Luke almost missed it, it was so soft. It was just a ghost of lips across his, and he wasn't sure whether it had happened or not. He had just come from blowing up the Death Star, when Han came in and saved his ass, confirming what Luke had always thought of him. He knew there was a decent man underneath all that bravado and Han had come back to prove him right. But even after the medallion ceremony, and the celebration that followed, Luke couldn't keep his thoughts from drifting back to that moment when Han had run to him and pulled him into his arms. Han's arms were strong and warm, and the love he had squeezed into him made his heart jump into his throat. Then he felt the barest touch of his lips against his skin and then it was gone, lost in the celebration. Luke wasn't sure what happened, and he knew asking was out of the question because he knew what Leia and Han had. It wasn't love but he was pretty sure it was a start.
The second time was out of spite. Leia and Han were arguing, he had just finished healing from the Wampa attack and he had been listening but trying to keep out of it. It seemed like a lover's spat, something he really wanted to keep out of.
"She expressed her true feelings for me." Han said, directing something to Luke for the first time.
"My...!" Leia was indignant. "Why, you stuck up, half-witted, scruffy-looking, nerf-herder!"
"Who's scruffy-looking?" Han pouted, and then turned back to Luke. "I must have hit pretty close to the mark to get her all riled up like that, huh, kid?"
"Why, I guess you don't know everything about women yet?" Leia smirked.
"Well, I guess you don't know everything about me either!" Han shot back, and leaned down and kissed Luke.
Luke was struck dumb by Han's audacity for a second before melting into it. No use not enjoying it. All too quickly, Han pulled away and left with a proud smirk. Leia turned to him and stared. Unable to keep the smugness off his face, Luke put his hands behind his head and grinned.
The third time it happened, Luke initiated it. The Empire was crumbling, without Palpatine the empire was flailing like a headless dragonsnake and the Rebellion had won the war. Finally, the people of the galaxy were free from tyranny and Luke couldn't feel more… happy. He couldn't help it, the loss of his father right after Anakin Skywalker had returned stung deep. He knew it wasn't rational, but he just wanted his father back. He barely knew Anakin, but he desperately wanted him back. He looked out over everyone dancing, and he heard a voice calling to him, and he turned to face it. There he saw three shimmering, smiling figures at the edge of the shadows: Ben Kenobi, Yoda, and Anakin Skywalker. His face broke into a smile to match his father's and his father gave him one last look before all three disappeared. But he could feel his father's spirit in his heart and for a brief moment, he felt enveloped by the love his father had for him, not the twisted and corrupted emotions that Vader had. Han touched his shoulder, concern etched into his features and Luke tilted his chin upwards to capture Han's lips. It almost wasn't a kiss, but an expression, a declaration. When Luke pulled back, Han was smiling and he just shook his head fondly at Luke.
The fourth time never happened. Luke had been wallowing in grief since Ben had forsaken his family to take a dark master and the name Kylo Ren. In his life, he had never felt more connected to Ben Kenobi. He knew the pain of losing a child to the dark side and he followed in Ben Kenobi's footsteps. He sent himself into exile, as punishment for failing the only child his sister had and for failing Han. Luke had vowed to keep his son safe and he failed. It was a sudden moment, when he felt Han's presence, even all the way out on Ahch-To. It was bright and felt like home, until darkness ripped it away and his heart shattered, a pain he didn't even think he could feel anymore. He fell to his knees, tears starting to stream down his face when the ghost of feeling brushed over his lips.