Why it's so rare that a player gives the 15s when the opponent is low on time?

When I have an interesting position, and my opponent is low on time, I often give them the 15s, sometimes more than 4 times. On the other hand, I lose a lot of games with a clearly "won" position on time, and there were just very few occasions that I received time. Is it really so much more important to win, even if you lose an opportunity to fight and to learn something new? Are people happy with a "win" that would result in a mate after a few more moves?

I just give time or give permission for take back a move in casual games...
In Rated games no excuses...

I'm sorry... but if you agree to play at a certain time control, don't be too surprised if your opponents stick to it. If you find it's not enough time, play with longer time controls (and especially an increment).

Landei, think about this. Had you used less time, would your position still be winning?

One game this dude and I were both low on time.

But the game was so good we both kept adding time for each other

@F_D89: Exactly!

@Ladei: Think of time as a resource, just like the pieces. Would you give your opponent back the pieces you take?

Players, even titled ones, have accepted draws because, although the player has an advantage, he/she has limited time.

Landei: 'Are people happy with a "win" that would result in a mate after a few more moves?'

Yes, much like a boxer is happy to win a split decision when in the last round his opponent has him on the ropes and is close to scoring a knockout, but the bell rings ending the round. If you are playing a rated game with a time control, winning on time is the same as scoring a checkmate, even if your position is lost.

That being said, as this subject has come up multiple times, there are degrees of sportsmanship that apply here, especially with longer clocks. For example, if the position is drawn and there is still plenty of time left so that a time out is not guaranteed, accepting the draw rather than playing to the clock is the appropriate thing to do. However, there are many players who will refuse the draw and just quickly push pieces around desparately attempting to win on time because they know the final stalemate position will not be reached for another 20-30 moves and they have a slight clock advantage. But in this case, giving your opponent time in a lost position that you will clearly win on time before they can mate you is not a matter of sportsmanship if the game is rated, it's simply part of the game. As Assisos said, the clock is a resource; it can be a weapon just as powerful as your queen, and it must be secured much like the safety of your king.

Sometimes I could add time to my opponent, but only in casual games. Now I started to do it carefully, because there was a lot of cases, when after giving time to my opponent, he didn't add me time too, nay consciously used it to win me by pushing pieces, like a mad. I don't respect such stupid blocks, so now I add time very rare, but when somebody gives me 15 or more seconds, I will return the same amount, definitely.