The Doctor didn't kiss River very much. When they kissed, she almost always initiated it. There were few, very few, times when he initiated the kisses. these are the most memorable. To both himself and River at least.
The first time was an accident. The Doctor had never really gotten used to those gangly giraffe legs of his. He regularly tripped over himself and others and almost always embarrassed himself in the process. Most of all around River.
He had been sitting in the library, correcting a large history book and muttering to himself when River arrived. She appeared, as she almost always did, with a smile and a "hello sweetie" that made His hearts jump.
He leaped to his feet in surprise with a strangled attempt at a hello and promptly fell over a pile of books straight into her arms. His lips crashed messily to hers and he struggled to pull away
He didn't recover for months.
The second of the incidents came when He had brought her to Paris for dinner. She had demanded A "normal" date.
He had tried wine for the first time and drunkenly kissed her. Afterwards he didn't even remember.
As the Doctor got older he became more comfortable around River. He no longer stumbled around her like a confused child and no longer blushed the shade of a tomato at every teasing comment she made.
He turned up at her house in the rain and pulled her outside. He was happy and so was she. They were drenched but they couldn't care less. His lips surprised her.
When the Doctor turned up at Rivers door with a new haircut, she wasn't surprised. Just happy.
He took her out and the Towers Sang, they danced together and he cried.
She could taste the tears as he kissed her to stop her asking why.
The fifth and final time the Doctor kissed River..well, it wasn't really River. Just an echo.
When the Doctor thought about it afterwards, he realised her echo was just as good a kisser as she had been...