Judging from the general lack of information on the web and the large variation of answers that I did find, I think we don’t yet know enough about dog behavior. From what I found, I concluded that our chocolate lab is looking for attention when he leans on you. (The other option was aggression and I’m not buying that as a cause for leaning for our 75 pound lap dog!) From Dominance aggression in dogs: Part 1.
Owners need to distinguish these behaviors from mere pushiness or attention-seeking. For example, if a dog is leaning against its owner just to get attention, the owner can physically move the dog without its becoming aggressive. Unlike most dominantly aggressive dogs, dogs that lean on a person for attention do not stiffen, open their eyes, and move with the person so they are again touching or pressing.