Headcanon is a truly disgusting, putrid concept. Actually that's not quite fair.
3/5 Headcanons Edit
Headcanon was originally used to sort out retarded, incomprehensible canons. This is pretty much needed if you want to make any sense of the Doctor Who Universe. The only other methods are living in denial or detaching yourselves from the universe and going back to being a member of civilised society.
4/5 Headcanons Edit
If you accepted the third option you may still encounter Headcanon for artistic reasons such as ambigous endings, implied info too subtle to get through TARDIS Data core's thick skulls or viewer interpretations in general. This interpretation is toxic and impossible to grasp for Tumblr people. Leading to posts like this (see right):
5/5 Headcanons Edit
The best headcanons are ones which actively erase shit from Doctor Who. This can include devaluing a Doctor you don't like by saying the previous Doctor's regeneration went wrong to explaining away stories to entire eras as dreams. This version is pretty petty but so is everything else on the site.
BASED WHITAKER/5 Headcanons Edit
-736/5 Headcanons Edit
Headcanon has since been abused and corrupted by the worst kind of Tumblrites to basically just make up their own shit and give it some kind of flimsy pretense of validity. The most egregious example comes from this blog: http://lgbtdisneyheadcanons.tumblr.com/ This blog isn't even related to Doctor Who (yet) but it serves as the perfect demonstration as to how you can basically make your fanfiction canon by just using your head, regardless of actual information showed, character consistency or unfortunate implications. This gets particularly irritating in Doctor Who considering how stupid the Tumblr section of Who fandom is.