Look, I am an atheist but I see value in asceticism and I see little or no value on knocking other people's values. Choosing to fast or give something up to remind yourself what you really have and what's really important doesn't seem like a bad thing to me. I'm sorry people have made you feel like you're ungrateful, I hope that isn't their intent and if it is, well, they should do some deeper reflection about why they're observing Lent.
My stance is that I keep my non-beliefs to myself. I won't preach to you if you don't preach to me.
The conversations I'm talking about tend to go this way (usually at work):
"What are you giving up for Lent?"
"I don't celebrate Lent."
*dirty look*
"Oh."
Or
"Did you have a good Easter?"
"I actually don't celebrate Easter."
*super dirty look*
"Why?"
"I'd prefer not to talk about it."
*ueber mega dirty look*
It's one of those things where, in the area of the south I grew up in, you have the older generations crying while watching The Passion reenactments and draping purple scarves over crosses. "Jesus died for your sins! Repent!" No, I'm good. You keep that over there.