Tears were a rare thing in Vegeta's life. Ever since he was a little boy, he had been taught to repress his emotions. Things like emotions were signs of weakness, and weakness was what got Saiyans killed. So, when Vegeta has heard of his planet's destruction, he hardened his heart and marched on. He would shed no tears for the loss of his planet and the people he was destined to rule. He took Frieza's punishments with dry eyes. During that portion of his life, Vegeta felt nothing but rage. But, no tears were spilled.
When Kakarot entered his life, Vegeta continued to feel that rage, but something new had emerged as well. After seeing how powerful Frieza truly was, Vegeta was filled with despair, and for the first time in his life, he cried. But his despair was replaced with hope when Kakarot arrived. Even as he was dying, Vegeta held on to that hope. He placed his trust in Kakarot and shed tears once more. Once he was resurrected, Vegeta swore he would never cry again.
Kissing Kakarot had already been nerve wracking enough, but when he felt something wet roll down his cheeks it became too much to handle. As Vegeta pulled away, he realized the tears were not his own. Never in the time he had know him had Vegeta seen Kakarot cry. The sight confused him, and he couldn't help but wipe the tears with his thumb.
"I'm sorry," Kakarot whispered, "I'm just...so happy."
Happiness. That was a feeling Vegeta hadn't felt until he met Kakarot. The man had given him so much, but Vegeta realized he had nothing to offer in return. Doubt began to form in his mind.
"You're happy...with me?"
Kakarot nodded, unaware of the turmoil going on in Vegeta's head. "Kiss me again. Please."
So Vegeta did.