Hi! My female lab is seen sometimes mounting/humping only on me while playing a little rough. She doesn't Mount on my spouse even though they play quite rough. What could be the reason for this difference in behaviour?

They try to hump on people who they feel they can dominate, they would not try to hump on individuals who they feel are superior and strict in their behaviour. This can also depend on the relative size of the two individuals or the way they interact with their pet.