Doogie's brain developed faster than his emotional development could keep up with, leading to insanity sometime after the show ended.
Later on, this mental instability led to Dissociative Personality disorder, with his Alternate manifesting as a mad scientist of sorts and ultimately taking over, leading directly into Dr. Horrible's Sing-Along Blog.
- Or House shows what an adult Doogie would be like
- Or conversely...
Doogie's mental instability led to him changing his name and personality...
His mental issues left him over or hyper sexualized with severe committal and relationship fears. However since he had the needed intelligence to have any job he wanted instead of continuing his medical career he quit to start law school, unknowingly joining Wolfram & Hart (as said in other WMGs), and had officially changed his name to.....BARNEY STINSON!!! Who didnt think of this Sooner??!!!
Also this goes along with the other WMG that Barney!Doogie is married to Maria Hill, and he exists in the Whedonverse in general