I have a four-year-old, very large (110 pounds,)very friendly Golden
Retriever who I take to places off leash almost everyday to play with
other dogs.  He enjoys playing with all dogs regardless of size, age,
or sex. However, sometimes a younger male dog (younger then four
years)wants to fight my dog.  Twice, the other younger male dog has
gotten very agressive with my dog.  If push comes to shove, my dog
will fight back.  Why do some younger males dogs and not other younger
male dogs get so agressive with him and want to fight?

Hello ddfmom-ga,

Let?s think of dogs like people. Get a group of people together at any
public place, say an amusement park. Some like to be friendly, others
like to provoke, and some lash out without warning when others try to
be friendly. Dogs are the same way. Some are very playful, like your
dog, and some are quite aggressive.

Some dogs have what?s referred to as Canine Aggression. It can be
dominance or fear related.

For example. My dog has fear aggression. He?s a little dog, so when a
bigger dog tries to play with him, he freaks out and snaps, thinking
the bigger dog will hurt him. You can read more about it here:
http://www.bcrescue.org/fearaggression.html

?In the canine the fear and aggression occasionally seems to "come on
out of the blue" but more often is triggered by getting into the dog's
"space" or protective territory.? Which your dog my being provoking in
some dogs while playing. Younger dogs have more energy, and are less
used to situation, which is why you may see this in younger dogs as
opposed to older.

http://www.thepetcenter.com/imtop/fear.html

If this answer requires further explanation, please request
clarification before rating it, and I'll be happy to look into this
further.
Nenna-GA
Google Answers Researcher

Google Search: dogs with aggressive personalities
://www.google.com/search?num=100&hl=en&c2coff=1&safe=off&q=dogs+with+aggressive+personalities&spell=1

You could ask the same about male humans.
The dogs your golden retriever meets come from/witrh different social 
backgrounds.  Maxbe the aggressive ones are the alpha dog in their
usual groups.

Their bad luck if the word gets back about their trying to take on a
110 lb Golden Receiver. (sound more pudgy than militant).