The training, if you want a nice sociable dog, with good recall you can't just hope it happens you have to put a lot of time and effort into it.
The walks nice in summer an utter ball ache in the rain and cold
Picking up dog crap while you're on a walk and having to carry it round with you until you find a bin
The initial pooing outside training, especially if you have children who go in the garden so want your dog to poo in a certain spot.
If you don't manage poo in a certain spot finding 'surprises' in your slippers while you're pegging your washing out, even when you think you've got rid of it all.
The vet bills that always manage to come in just below the insurance excess
The cost of insurance, food and treats they can cost a decent amount
Being tied to days out and places you can take your dog or having to arrange kennels for holidays
The chewing especially puppies they chew everything and anything
That's everything I can think of.
My mum and dad always had dogs while I was growing up and still do and while I love/d them I couldn't have one myself.
I'm constantly borrowing my mums dog to take on walks as it gets me out but I'm glad that I don't have full responsibility for a dog.
I think you have to REALLY want a dog before buying one because otherwise the novelty would wear of quickly, which is probably why there are so many dog rehoming centres.
There's a page on Facebook called walk my dog or something like that where you can walk and pet sit dogs who belong to people locally. It saves them paying people to walk their dogs and lets people walk dogs and not have responsibility for ownership, you could try that first to see how you find it if you're unsure.
I feel like my post is negative it's not intended to the positives are that they do become part of your family, have their own personality and are lovely company