That tear is for everything and everyone The Doctor has ever lost. It is for his family, his friends. All the people who have ever sacrificed themselves to save him and all the people and creatures that he has ever killed.
That tear is for Amy and Rory
for Donna and Martha
for Rose and Jack
for Clara and River
It is for all the friends and companions he has ever lost. For everyone who has ever said goodbye. For Susan, Barbara, Ian, Vicki, Steven, Polly, Ben, Jamie, Victoria, Zoe, The Brigadier, Liz, Joe, Sarah Jane, Sergeant Benton, Mike, Harry, Leela, K-9, Romana, Melanie, Ace, Micky, Astrid and Wilfred,
It is for all the lives he has ever touched, all the lives he thinks he has ruined. All the people he has ever loved. It is for the entire human race.
He is a parent who has lost his child, and he’s not hiding that anymore.