I miss my first bike, 82 MX 100, It would be a great girl friend trail bike, At the time I sold it for an upgrade so no real regrets.
My IT 250 AKA the Iron Horse, was a great bike, I beat the poo out of it real hard and it always ran and I put very little $ into it. I think I sold it for more than I paid for it 4 years later :). Its still not as good as my spare bike 89 YZ 250 wich owes me nothing as well so I dont regret selling it.
If I ever sell the WR 400 I may regret it, unless what ever I get next is WAY better.
I suppose its important to sell your beloved bikes for even better bikes and not cause you need rent or a dumb house or a car or somthing,,,,, that way there is peace not pain :) .