A dog is 24hours a day 365 days a year. They are expensive: flea treaments, vaccinations, food, treats, equipment, harness, leads, training classes, castration/speying, insurance etc.etc.
Constant grooming: ears teeth, paws, eyes, daily cleaning of pawprints/dog hair, picking up poos, anal gland attention. Daily walks: not because you fancy it every time but because you have to, whatever the weather. Behaviour issues: barking, biting, jumping up, seperation anxiety, destructive behaviour, chewing, toileting in the house.
No spontaneous trips out/weekends away. Not fair to leave them for hours each day. Maybe lifetime medical conditions.
And sometimes there are upsetting walks due to your dog being attacked by another, or disappearing after rabbit or squirrel or cat, or running into a road.
I've had dogs in the past and loved and enjoyed them so much. But decided not to have another now for a couple of reasons.