The biggest problem with big dogs is that when we owners get older & have issues (like knee problems, etc) those big dogs are just too much to pick up when (not "if" but "when") they get sick & need to go to the vet. Sure, when they're healthy & younger, they'll hop right into the car when you ask them too. But when they're sick or injured, that's a whole other thing. And if you bathe your own dogs (as I always do) that's another time you have to be able to pick them up.
An older guy that I swim laps with- his best ever dog was a Golden Lab that he had for years. When she passed, he decided to get another Lab, not realizing that the personality of the first dog was NOT typical for the breed. They're not known for being mellow as she had been, so now here he is, up in years with a high-energy lab! And his wife is very petite, no way she can control this dog if she ever has to. He won't give up this dog, but he knows now he really goofed when he chose her, & it's not that he doesn't love the dog...she's just not a very good fit for what he needs now.
There would be fewer dogs taken to shelters (or otherwise given up) if everyone would give a lot of thought first, to what they really need & want in a dog- don't just fall for the cutest face or the breed image that will impress your neighbors/friends. I've had all sorts of dogs over the years & I truly like all kinds, but I know what fits me now. (These 2-)