The Doctor didn't seem to have a point any more, and it was all his last self's fault.

It is true that the Doctor never knows what he's going to end up with in the regeneration process. Being half-human messes up the natural capability of Time Lords to control their next self's appearance and personality and whatnot. But there's still that tiny ounce of control, and when exacerbated by a remnant of the Vortex...

Nine hadn't been intentionally mucking about with his regeneration. He just wanted Rose safe and happy and preferably with him, and that inner instinct took over.

It took a while for it to properly sink in, but he finally figured out why he had so many characteristics which he hadn't had before. Rose got uncomfortable when Nine went off on his sulky silences, so Ten got the amazing ability to talk pointlessly for hours on end at speeds previously unheard-of even for him. Rose continuously brought pretty-boys onto the TARDIS, so Ten got to be a pretty-boy. Rose never quite seemed to tire of anything until long after Nine was bored out of his mind, so Ten got more excitement about the most mundane of tasks than either of them knew what to do with, as well as boundless energy. Rose got into trouble almost more often than the Doctor himself, so Ten got the inability to do anything useful until after Rose was out of danger— and the ability to intimidate the hell out of whatever had so much as looked at her the wrong way. Rose was obviously the most intelligent and resourceful creature in the universe, so Ten got the inability to see anyone else as being anywhere near her equal in those categories. Rose had immensely enjoyed Nine's method of saving her from the Bad Wolf (He did have to look into her thoughts for a bit so he could lock the memories away, so he got an excruciatingly clear sense of precisely what she thought of the moments just before she lost consciousness), so Ten got to have an oral fixation and the almost irresistible urge to use it on her and not just random bits of technology.

In short, every nuance of Ten was there just to keep Rose safe, happy and with him. He was designed for that purpose and no other.

So when she was gone...

Donna couldn't stand his tendency to babble pointlessly when they were in the middle of danger.

Martha got bored far more easily than Rose, and was also extremely irked whenever something even slightly disgusting happened in her general direction.

Astrid... well, he hadn't really hung around her for long enough to figure out how she didn't fit. He just knew she didn't.

Donna's tolerance for his nattering rose, but she cultivated an intolerance for his automatic assumption that she was mostly useless.

Nobody fit into that hole Rose had left. Nobody tolerated or liked all of him. Try as he might, he couldn't find anyone to revolve around who wasn't in Pete's World.

He was the Oncoming Storm, but nobody but her could safely get to the eye and be protected by the Storm which raged all around it. He continued to destroy everything in his path, but for what purpose?

None.

He was pointless.

And it was all Nine's fault.