Identifying why a change in my dog's behavior

I’m a bit stumped on why my dog is becoming defensive/aggressive towards some people and other dogs.

Chocolate Lab
5 years old male
Neutered

Last 4-1/2 years the most loving and cuddling dog. 105# lap dog that just wanted to lay next to you or on you. Absolutely had to be in the same room and interact with you.

Over the last 6 months, I’ve been noticing a gradual change in his behavior.
Few concerning and key things:
1. Previously passive towards other dogs, now growls, barks, and stands the hair up on his neck/back and immediately shows his teeth.
2. People he is accustomed to being around on a regular basis, has now growled at. This includes my wife.
3. Still is very excitable for playing fetch. But is becoming VERY independent/removed from interacting with us. Will now go lay by himself in a different room. Had been nearly a lap dog, now there is an obvious separation. He still likes to be petted, but Just a little bit then he gets up and walks away.

No signs of injuries or any other issues. Vet says he is 100% healthy and in awesome shape. No changes in our household schedule or routines. No change in his diet, eating habits, or anything I can identify.

Knock on wood. Our dog did the same thing late in 2013. He was 4 then. Became extremely protective of my wife. Starting attacking other dogs, snapped but never bit our adult son who lived with us, etc…..in early 2016 my wife was diagnosed with cancer. A fist sized tumor was removed from her chest, did the chemo, two more minor surgeries and is in remission, doing great now. We do wonder if it was all tied together? Did the dog sense her illness? Doc’s say the tumor could have been growing for years. He is better now but we still don’t trust him around people or other dogs. He has not snapped at anyone since the wife is better.

If you can rule out health issues, then my next guess would be something hormonal with him. I’m not sure if dogs go through hormone changes as they age, but maybe there’s something he’s not getting now in his usual food/routine that his body isn’t making as well as it did before. Maybe try switching foods or asking your vet about a vitamin supplement of some kind?

Perhaps it’s a smell in the house that he doesn’t like. Did you change laundry detergent, air fresheners, perfumes, or colognes? I’m not sure if this would have much merit but it’s worth a thought.

I have heard of dogs acting strangely when the people they’re close to are sick or pregnant, so hopefully that’s not the case.

Hopefully, nothing with a family member.

Sharon, vet did blood work. Not sure what all they test, but said he is very healthy. No other changes that we can figure out…

He does still think he is part human and sits in HIS chair

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I have a lab that kind of went through a phase like that as well…around that same age he started acting out a little…nothing changed in his routine or anything and he is not neutered…luckily he grew out of it…kind of reminded me of a rebelling child…shorlty there after I got a female and she did get fixed and he sort of grew put of it and went back to his old ways of being a lap dog…basically an overgrown baby…hopefully it’s just a phase like it was for my male and he will come around…my female on the other hand…sometimes I don’t think she likes herself she’s quite the diva

My fist thought is to have his eyes checked.