Why is Alan considered to be a loser?

It's because the show constantly places him in line to be the hapless character for the sake of comedy.

The main activities which define this are

• Divorced and paid alimony
• Son doesn't respect him
• Lives at brother's place
• Controlled by his mother
• Berta (the maid) doesn't respect him
• Falls for the wrong women

Then through the episodes we see that he always encounters the unlucky situation and the audience becomes desensitized to the failures that happen even though it would be gut wrenching for this to be real life.

I would like to say it's a continuous vicious cycle of redefinition and strengthening of the definition of the name calling that pushes Alan to believe he is a loser more than he really is. And thus, it becomes more likely for future issues to continue to blow up in his face due to the negative setting he was previously in.

In reality, a person would be able to eject from this by getting help but as this is one of key devices of the show, Alan Harper must remain the loser.

Cause Charlie Sheen says so :)

On a rather serious note. I think divorce has got everything to do it. Let me put it with a realistic example if you look at current scenario of the events which unfold in modern day life it doesn't matter what you're in the past it always matter what you're in the present (Best example probably would be sports world i can cite several instances from it). In the Two and half men scenario he is shown as the guy who stays with his son at Charlie sheen's place so that's the reason he could be pictured like that and added by Charlie sheen and his son's mockery doesn't help his cause at all.

The main person that calls and treats Alan as a loser is/was Charlie, and that was primarily because Alan is a moocher/freeloader. That is, even though they are brothers and Charlie would have been happy to help Alan out for a while, Alan took advantage of the generosity and permanently moved in.

Even worse, Alan was cramping Charlie’s lifestyle which of course, Charlie was upset about.

As a result, Charlie constantly complained about Alan in the hopes that Alan would take the hint and or be offended enough to move out.

When Charlie died, Walden bought the house. Alan saved Walden’s life, so Walden felt obligated to help Alan out a bit, but once again Alan took advantage of the generosity and basically moved in (or rather, did not move out). Walden does not insult Alan as much as Charlie did because they are not as close, so Walden feels awkward insulting Alan who he met less than a year ago.

Jake insults Alan because, well, he’s Alan’s teenage son and unfortunately that is typical of many American children.

Alan’s ex-wife Judith makes fun of Alan as well, but since she left him, she presumably has reasons to dislike him. Plus, she has to constantly fight him for alimony and child-payments. She has since re-married Jake is now an adult, but she is likely to continue to hassle him for other, personal reasons.

Finally, his own mother treats him poorly, but then Evelyn treats everybody poorly.