"The humans specifically?" wondered the Doctor. "Why?"
"Well ... maybe it's because this quadrant is a harsher place than the Alpha Quadrant seems to be - but humans have a positive mindset I never encountered anywhere else. Even the grumpiest, crankiest, half-Klingon or holographic human seems to believe everything will come out right in the end if you just stick to your principles and apply yourself.
"That's a reflection of my own outlook that I don't see very often. I don't know if you've noticed, but many people consider me annoyingly cheerful."
"It has on occasion just crossed the threshold of my perception," admitted the Doctor.