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Okay, so I've read my fair share of fanfiction on this site, but I never made an account because I didn't need one before. Now, sometimes I wake up in the middle of the night with the need to write the next great American novel, and apparently sometimes I wake up with an MLP fanfic. I just drag myself to the computer and go with what I get, squinting away from the glow of my monitor in the middle of the night because sometimes stories just need to be written when they need to be written. In this case, I dig through old google docs accounts looking for a story that first took me in the middle of season 2, and when I find it I give it a good spit-and-shine. Suddenly I have a story to share, so I make a FIMfic account, formatted the first chapter and let it fly.
I don't feel entitled to be read. Even when I'm selling stories for money, I'm not entitled to sell any. No matter how much work and heart I put into a story, no matter how much I love it, everypony has their right to be fickle and either ignore it or scoff at it as they will. Because I have no right to be read and potential readers have every right to not read me, it means that much more when I do get read. Therefore I am really eager to have my first reader. So before I go back to bed, I have to sit and stare at the screen for awhile. I know that FIMfic has an approval delay, but I do this with everything, whether it's just a flash fic or an RP post on a forum somewhere. I like my words, and I'm desperate for anypony else to give them the time of day.
Eventually I did get readers; three favs, actually. Three precious, beautiful favs on my first chapter. Before I got any of those, though, I received a comment on my user page from someone asking me to go and read a story they'd just posted and give them some critique. They furthermore asked if I'd let them know how I felt about receiving their copy-pasted comment, and if I considered it spam. This blog post is to respond to that person, but not rudely.
There's absolutely nothing wrong with requesting critique from your readers and friends. There probably isn't even anything wrong with asking for critique from strangers who are in the habit of handing it out. The problem with what you did when you posted this to my user page is two-fold.
One: I have posted a story to the site, which you did not read. You should never, ever ask another writer to critique your work if you have not first offered to critique theirs. Give before you receive.
Two: I had been a member of the site for less than a single day, had no followers and had never made any comments on anyone's stories. This means that not only did you not even consider reading my story before asking me to read yours, but you didn't even look at my user page to consider if I would give worthwhile critique or not.
There's a reason that professional writers join guilds, and there's a reason those guilds have critique circles where everyone who attends offers critique for the stories of other attendees. One thing that outsiders may note about those critique circles, if they were the guest at one, is that not all of the writers offering critique are seeking critique, but every single writer who is seeking critique offers it to everyone else. I know a romance mystery writer who has been attending the same critique circle every single week for almost a year and has only brought her own work one time; this is an extreme example.
The point is that if you don't give critique, you don't get critique. What you did when you posted on my page blindly was, yes, spam, and it was also terribly ineffective. Critique is useful because it helps you improve a story. Offering critique is useful because it helps you improve as a writer, making everything you write from then on better, forever. If you really wanted to write great stories, you'd read and critique as many stories as you could, including ones that you wouldn't read otherwise. You would find stories that you don't necessarily enjoy as a reader, but that have flaws which present lessons that interest you as a writer. And don't assume that this is not a method that has unspecific, amorphous long-term results. If you start giving honest critiques today, you will begin writing better stories immediately. There is no delay, and there is no limit, and there is no down-side.
It would also get you way more comments and critiques then posting blindly on the walls of people who have only been members of the site for an hour. The one rule of thumb that I give every writer, whether they're professional, amateur, fanfic authors or role-players, is this:

You will begin to succeed the moment that you make the success of others your primary goal.