My 6month old Bull Mastiff is trying to hump my 4month old Lab x in a agro way.....?

And yes, it’s perfectly normal, if more than a little annoying. Since dogs are social animals, one of the things two dogs in the same house will inevitably do is to self-define their social order. Your older pup is telling the younger one that he is top dog. That’s all it means. As the lab gets older, this behavior may switch around. Both labs and bulls are tough dogs, and they’ll probably tussle this out over and over again. Personally, as long as no injuries or TOO much rowdiness are happening, I’d let them figure this out for themselves. Getting in the way of the dominance struggle CAN lead to nastier behavior for the “protected” dog. Unfair as it may seem, reinforcing the top dog’s position is actually helpful in many cases. What you DO have to watch out for and stop IMMEDIATELY is either dog trying this behavior on YOU. YOU have to be the “Top Dog” for this pack, and such behavior must be instantly and firmly stopped. We prefer the “Pick up” method—we do NOT advocate “rolling and pinning”. If one or the other dog tries this with you, straddle the dog (your head and the dog’s head facing in the same direction) and lift his up with your hands under his chest, so at least the front legs are off the floor. Hold him like this until he stops fighting you. Put his feet down. If he tries to fight again or to get away, pick him us again.