male dog behaviour

Question
QUESTION: My 5.5 year old unneutered male labrador (a really smart, friendly and gentle dog) for about the last week has been trying to mount my 3.5 year old neutered German Shephard (neutered over about 1.5 years ago). Do you have any idea why he might behave like this? He is not doing it all the time, just periodically throughout the day. Thank you for reading this question.

ANSWER: It is generally dominance-though getting him neautured may reduce it,not because it reduces a dog's sex drive(dogs don't really have one outside of breeding a female in heat),but because it reduces dominance

But it is him trying to be 'top' dog,it will be reduced over time as he learns his place

---------- FOLLOW-UP ----------

QUESTION: Thank you for reply. Could there be another reason? He is not really a dominant type of dog. He has always let the two females eat his food if they finish first and I am not watching, take balls/toys/sticks from him, playfully nip at him (they small one swings out of his neck).
He actually does not get up on her from behind. He sniffs there but usually tries to get up on her side!
WOuld he suddenly try to show dominance after years of being with the other dogs?

Answer
Sometimes,over time and for reasons really unknown,sometimes dogs that are not in charge,want to challenge the dog in charge,usually by settle ways such as this.I have seen this  anumber of times

The only other thing is there could be a female in heat not to far away,and it could be 'exciting him'.That can happen as well,and once the female is no longer in heat the dog will calm down