Wanna know what really happened on the night that Sasuke left the village? Fine. Sasuke and Sakura will tell you.
After Sakura had confessed her love for Sasuke, she tried to make him stay by asking him to kiss her. This is how it all happened.
"Sasuke! I love you so very very much Sasuke! I wish you could stay with me! I promise you! You will be happy! We'll have fun every day! And you will be satisfied whether you like it or not! I'd do anything for you Sasuke! So... I'm begging you don't leave me! I'll help you with your revenge. There must be something I could do! So... please, Sasuke. Stay here. But if that is not possible, then please take me with you."
Then, Sasuke said:
"Like I said, Sakura... you're annoying!"
Sakura then said:
"Kiss me, Sasuke!"
"What?" asked Sasuke.
"Kiss me." said Sakura calming down. Sasuke then walked forward to Sakura and gave her a quick kiss on the lips. "No... I mean, really kiss me... Sasuke." said Sakura. Sasuke then started kissing Sakura passionately and he ended up sleeping with her that night at his house and he never left the village again.