I sometimes want a dog, too. I imagine myself happily walking with it and giving it treats as a prize for doing some new trick.
THEN...
I think about having to wake up in the middle of the night to let it outside to go to the bathroom. I live in a house with a yard, so it wouldn't be so bad...but in an apartment situation I would imagine you would have to keep the dog on a leash and stand there while it does its business.
I also think about it teething and chewing all my stuff! My friend's freakin' dog chewed 2 pair of my shoes and I didn't even live in the house with it. It just chewed everything. One day she came home from work and it had her new MAC lip liner chewed up all over her leather sofa! Looked like a 4 year old was drawing all over the sofa. I'm telling you - thousands of dollars in chewed up stuff!
Then, what if you don't get home in time for the dog's bathroom break? It might go to the bathroom in your apartment! Then you have to clean your floors right away or your place is going to stink! And, being 21...I would imagine you may be 'entertaining guests' so you don't want your place to smell like a kennel.
Shedding could be a problem too. So, either buy tons of tape or a bunch of lint rollers to have handy. I can't stand seeing dog hair on clothing - it's gross really.
Then there is the barking. What if ya get a yapper??? Your neighbors will love you.
And finally, I think, there will be dog toys all over the place. And they are slimy slobbery - yuck.
No, more:
Fleas.
Vet bills. You don't want to neglect it's health.
Food costs.
Accessories.
Just sayin'... I shouldn't own a dog. You should think about all this stuff and if these things wouldn't bother you then go for it. I got a book once, "Woman's Best Friend" which lets you answer questions and it steers you toward a dog that would be a good match for you. Maybe there is some online thing that would serve the same purpose.
Good luck!