one of the great unexplained mysteries of spn why did they dub this from "love" to "like" i mean. they wrote, filmed and edited it and then... the network i assume. the network....
Birthmark is such an interesting episode beyond what we learned about House's family. It also told us so much about how House and Wilson both decided to choose each other as their family and "birthmark". House had a lousy family, so he seek out Wilson on purpose, a person who has trouble losing people, would thrive on his neediness, and be there for him all the time. It's almost also out of such kindness that House offered Wilson his friendship, so Wilson never need to face what to do if House no longer needs him like all his ex wives did.
It explained how they found each other in the first place, and how they decided to choose each other again despite the great tragedy they faced together. They both know each other better than their family members. We never even ever saw Wilson's parents, so they probably never needed this son in their picture; neither of House's parents knew him that well either. When House's mom said "I knew you'd make me proud" and Wilson replied "I'm sure you know him better than I do", House looked back looking like a hurt kitty. And when House was scheming to get his father's DNA, it was also Wilson who immediately got suspicious meanwhile his mom only looked moved by the speech. House also talked about how Wilson was breaking up with him only because Wilson was scared to face losing him. This further explained the hurt kitty look on Wilson looking at House after Amber died. He was so regretful that his enabling of House and his friendship with House almost killed two people he loved most in the process.
They truly chose to reborn as each other's family and later chose to die together.