My puppy is being neutered today....what do I expect?

It depends on the dog.

An empty dangling sack?

The vet should explain things before you take the pup home. It's normal for them to be tired and a bit all over for the rest of the day/night. You should be given a pain med for the pup too. He may not eat at all until tomorrow.

How they behave after will depend on the puppy. As with humans, each animal varies in how things affect them. It may not make any difference to behaviour at all.

Maybe a cranky puppy in return?

No idea what to expect after he's neutered...

But my current dog was done by the time I got him, the main reason I got was that he's too Alpha and was to prevent aggression.

But now he's 2 1/2 and he's the most mellow dog ever, he pays no attention to female dogs even if they are on their period, he'll submit to other Alpha's quickly so they can play rather than fight... temperament wise my boy is as close to perfect as you can get really, that's what everyone that meets him thinks, he's spoilt but so obedient.

If the neutering had a part to play in that then I'd say you can expect a cooler dog thats more concious than a dog who is still controlled by the natural urges caused by their balls, lol.

I'd like to think I've had a positive effect on him as well, apart from the dudes that take their dogs skydiving or surfing or something, I think I'm giving him an awesome quality of life and he knows that.

Resentment

Very pissed off. I would be if you cut my balls off for your own personal sense of enjoyment of having a pet.