Disqualifying the positive
Some people simply dismiss positive events for no good reason, probably while being all too eager to accept the negative ones.
Have you ever had a conversation saying that you had a really nice conversation with Miss X at the sports event and came a reply “But it doesn't count. She's just friendly to everyone”
When you told someone that the guy complemented on your work and the response was “He doesn't know what he's talking about.”
Or when you were telling someone that your colleague has changed for the better and that someone responded “I'm sure he'll be back to his old way soon enough."
Slapping simplistic labels on things in order to explain them, rather than looking at the unique facets of the situation is known as labeling.