My puppy didn't sleep through the night so 1-2 times he needed to go to the toilet. Sleep deprivation was a killer.
The biting was hard. I had holes in my jeans where he would jump and hang on.
The manic running about in the evening. Puppy zoomies are wearing.
The constant list of things you have to do for training and socialisation. Trying the get him to sit to groom him. Trying to get him to walk on a lead and not be scared of the dark. Cleaning his teeth. Manipulating his paws and claws to get him used to being manhandled. It just felt endless and that is before you get to the simple things like 'sit'.
The constant accidents just when you think you have cracked toilet training. You turn your back and there is a puddle. The constant standing in the freezing garden waiting for them to toilet so you can praise.
The watching and constant supervision to make sure they are not eating the garden plants, my shoes, the rug etc.
Then we get to teenagedom. The fingers in his ears as he obviously can't remember how to sit and stay and what was that about recall. Oh yes recall means I will come back in about 15 mins when I have finished having fun in another field.
I am sure I have missed lots 
We are on puppy survival thread number 5 on here. I joined the first one. You might want to read all 5 to remember what it is like 