Tracer Headcanon: When she disappeared in the Slipstream accident, Tracer witnessed the entire span of the universe, from birth to death, in real time, as a kind of intangible, omniscient, unfeeling observer. She hid this from the Overwatch psychiatric team in order to stay on active duty. As far as they know, she just has incredibly debilitating depression. She hasn’t given up on Widowmaker because she knows exactly what it’s like to be utterly numb to chaos and atrocity.
YESSSS GIVE ME ALL THE TRAGIC TRACER HEADCANONS.
Honestly, though, perpetually and overly cheerful people are typically manifesting the behaviour to hide other emotion and make themselves feel better (been clinically proven to work, btw!), and because making other people feel good also makes them feel better. Typically, though, they’re either currently depressed or previously depressed.
And yes, this is why I think both Mercy and Tracer would be invested in Widowmaker’s recovery - because in Widowmaker they see a darkness they themselves know very well and have overcome.