Why do parents rant about ungrateful kids when they decided to have kids in the first place?

I personally feel like if anything, they should've expected it to happen at one point of their lives. Parents shouldn't expect their kids to understand how much they sacrifice for them. For me, I'm not a rebel; I'm a churchgirl and my mom constantly tells me how ungrateful I am. I may not even be doing anything to annoy her and she talks about how tired she is of me asking her for anything. The messed up part is that sometimes she offers to do stuff for me and complain about it. How are you giong to offer something and then complain about it? One time, I was dressing up for a basketball game and she didn't think I looked good in it so she offered to take me to the store to look for another outfit. Once I tried on some tops, she was already like "Courtney, I don't know why you're into this mess! This is too much!" For real? I wasn't even getting an attitude with her. What teenager want to hear his/her parent complain about giving their kids something, especially when they offered? Isn't parenthood supposed to a good experience? My mom doesn't mind spending $100 in one setting on my younger autistic brothers yet she shows irritation when I ask for just $25. I feel like I'm the black sheep of the family. I've always felt like she favors my brothers over me no matter how often she says otherwise. Also today, I was at the Proscout interview session and mom said that I could go as long as she wasn't involved. Lo and behold, I found out that I had to be 18 in order to be interview by myself. My mom flipped out on me and talked about how much her life sucked and how I'm making it so hard for her. I was already pissed that I couldn't be interview by myself because I really wanted to be an actress and her rant added to the negative emotions that I already had in the first place. The last thing I needed at that time was a guilt trip. I'm really convinced that she hates being a mom. She complains about how all she does is give give give. Well....that's what it takes to be a parent. Once they have kids, it's not about the parent anymore. All I'm saying is if you can't handle it, then don't have kids.