Let us stop being a boastful giver (GA included)

I never expect a winner to leave a thank you, but it is always nice to see that the winner appreciates the game.

I Expect the world...

If I'm giving away a shitty game I don't really care if the winner doesn't thank me
but if it's a decent one, it's nice to read a thank you (´・ω・`)

I don't expect a winner to say "Thanks", but it's pretty much just good manners to do so. I would feel remiss if I didn't let someone know I'm grateful for a game I've won.

And honestly, I see a lot more posts of people complaining about the people who do say thank you (notifications, blah blah blah), which is just completely foreign to me ...

It's always nice to see a thank you message, but I never expect it. I myself forgot to leave a "thank you" note a couple of times, so I can be called guilty of that crime :)
For me it's basically like you said:

we do it out of the joy of sharing, without any expected returns

It should not be obligatory, but I like to express my gratitude to the person who's giving me the game.
I don't like the generic "thank you" and prefer to write something from the heart.

I would say: Less spam, more love!

Just one of the reasons I stoped doing public giveaways

I stopped writting thank you since a lot of people started blacklisting you if you comment just with "thank you"

Say "Thanks!" every time and get called a bot, stop saying "Thanks!" and get called rude.

Damned if you do, damned if you don't!

I do expect a winner to be grateful. Sure we gift because we want to gift, but we also gift because we want someone who would be happy with the game to win it. And if the winner is happy to win the game, then it would be nice of them to acknowledge that.

But that's just in theory. In reality SG is just a random gift trading site, not something where you would expect a winner to think more than "okay, I got another game for my collection, now I need to gift something in return".

Look at the other way,

Why many people are usually doing group/whitelist/private giveaways? Mostly they want to know the other one is a nice person (or one that they can get along) otherwise every GA would have been public.

So looking at this light what does it tell you about a person that does not even bother it. It is not a thing gifter "needs", it is lack of simple "common courtesy".

When I do public giveaways, I don't expect any of thanks comments from winners except it requires to be friends in steam.
I understand it for the very low rate at winning the public giveaways with no restrictions. It's like against a large populace who doesn't see us as a single person.

I would say a thanks should be given, as a token of appreciation for what the other person did. That's common decency and although it's just a small thing here in the grand scheme of things, it's a start and the more decency there is in the world, the better.
However, I would not go as far as expecting it in a way of "If you don't thank me, you're going on my blacklist and I will re-roll".
Mind you, the lack of private messaging here on the site made me not thank people at my earlier wins, since I wasn't aware of the possibility, or method, of posting in the giveaway thread, so I could only thank people that contacted me on Steam or via mail to pass the gift to me. Maybe that is also a reason why some people don't thank. They might not know what the procedure for it is.