The second time, watching a good friend age and die before his eyes, he still couldn't adapt.
The third time, he developed an urge to flee, thinking that if only he would not make new friends, if only he wouldn't become too close to the friends he did happen to make.
When friends aged and died, he wouldn't feel sad or heartbroken.
He failed. If he were like other Pureblood Dragons, who could fall into a slumber for hundreds of years at a time, then perhaps he could have managed it.
Unfortunately, he wasn't that kind of dragon; he couldn't bear loneliness and loved to learn in the Human World, loved to make contact with agreeable humans under a variety of identities.
This condemned him to experience the sorrows of parting and death.
Gradually, he figured it out; he let his good friends send him off, watching over him as he was laid to rest, spending money on his funeral.
Before he turned nineteen hundred, he would see his human friends off, watching them being laid to rest.