How come when at the third part of do wewe want to build a snowman, Anna asks Elsa to build a snowman with her when there is NO snow?

She was only asking because she remembers the fun she thinks they always played in winter but they actually played in the ngome ballroom in July when they were little

She is depressed and just rembering a kind of good memory of the song.

The troll people only got rid of the memories with Elsa's powers in them. The other ones where they played in the snow she kept. As for why she's asking when it's not snowing... I don't know. It's probably as meggy24357 said, "good point maybe it just became a habit".

Anna wanted SO much to play and spend time with her sister, her only loved one/family member in her life and vice versa with Elsa... but Elsa felt like she had to continue listening to her parents and conceal/don't feel.

She was practically saying that because it reminded her of fun times with Elsa.