I don't understand why this person did this.

I recently played a game in which I was down significant material and was going to be mated soon. In the chat my opponent said, "should I be nice?" I assumed he was either going to be nice by mating me, or be not nice by prolonging the game. I said "your choice." He then promptly sacrificed his heavy pieces and I ended up winning. I asked why, and he replied, "because I felt like it." I have never encountered this before in my years of playing chess. I'm not sure what his intention was, but I ended up feeling ashamed. What is the psychology behind deliberately losing a won game? Is this a common occurrence on this site? Was it some sort of perceived altruism on his part? I'm very confused.

Chatting is a noob’s quirk. Just play.

(chatting during the game. Before and after: sure, feel free!)

There is an easy explanation: We have a big number of people here. Even if 99% of them were quite normal and respectable, there would be a rest of several thousand other ones with - err - strange opinions and behavior patterns. E.g., they don't want to play chess, they just want the others to feel bad.
Ignore them.

Edit:
Report him! Deliberately losing is against the TOS.

See, I'll deliberately crush you like the dawn crushes your dreams (and they are forgotten before they are ever remembered) then linger as long as you do and make casual chat

__
Do you need to know why to accept that not everyone is gonna provide competition in good sportsmanship? People are ugly and there are an infinitude of causes for this. They can't help it. These people are suffering and they are reaching out in pain with pain and no one is going to save them. Because people don't give a fuck about each other.

>here<< just below here

lichess.org/forum/redirect/post/thqlrtCg

to what I think is wrong but it's listed officially as poor sportsmanship but is not fullysupported by state and local authorites. yes

some people are just overall nice i recently played one in which i was winning but i lost connection and instead of him claiming victory he took a draw and when i came back i realized i only lost 1 rating so i went back to check what happened and found in the chat he said " lost connection bro?", i find that to be nice

i hate that kind of things too, never happened in a chess game but i often see that in card games (like rami,poker etc...). these people don't understand what they are playing, they don't understand these games are all about competition and being nice is being mean. It destroy the interest in the game, you feel like you wasted your time. that's cruel, you were having fun just a moment ago.

i advise you to just play a lot in the next few days, that will help to forget this ugly game. Also, ban/block that player so you will never meet him again.

Sometimes boredom players a factor

Ive done this before; simply tohave fun, or because Im innebriated; Especially in Crazyhouse.

Gimme that pawn for my queen
Then I still win anyway lol

I suppose he had achieved what he wanted - reaching a won position. Rating points aside, his satisfaction and enjoyment are sufficient at that point.

Perhaps - if the game was rated - he doesn't want the points. It can be more interesting and fun (if less challenging and satisfying) to play people a bit less able than yourself at times rather than constantly struggle against equal or stronger opponents.

Or perhaps he didn't want his rating to increase, for other reasons.

There are people who sandbag, AKA, deliberately lower their rating by losing lots of games.

It lets them easily win capped tournaments.

Not saying he's a sandbagger, but there are people who will let you win in winning positions to lower their rating.

As sheckley666 said, these crazy players must be reported.
But I use to block people whose unsportsmanship behavior piss me off.
Last week I lost a game (a very good and interesting game, by the way) to a guy who, in other circumstances, I would gladly play against more and more games - but unfortunately, he was too cocky, too talkative, too offensive in his language in the chat, that I had no choice other than to block him/her.
(Sorry for my poor English)