Why do some adults still act like children?

Yeah, I have a SIL who I'm wondering the same thing about. She's 25, I've known her since she was 13, and she's actually actually less mature now than when she was 13.

She got mad at my husband and I because we chose Owen for our baby's name, when she told us the day before we were finding out the gender that she and her boyfriend picked ""Ewen" for a name for a future son. Owen and Ewen are too close, in her opinion, and so she asked us to "keep that in mind" when we chose a name. We chose Owen anyway, and she went crazy. Keep in mind, she's not pregnant, and has had a miscarriage because she has health issues, so until those issues are sorted out, she can't even attempt a pregnancy. But she's angry enough that she called me crying and screaming at me and told me she they can never forgive either of us. She's deleted me and blocked me from facebook and refuses to speak to me now. We were both at her parent's house last night and she wouldn't even come in the living room knowing I was there.

Actually, to be honest, after 12 years of pussyfooting around her, I'm relived that we don't have to keep up appearances anymore. My husband has tried talking to her about it but he turns the story around and claims I said something about her miscarriage that I never said...trying to turn him against me. Thank god he's too smart for that.

My husband also doesn't think she's mentally stable enough to be in the same room as our kids alone. And last night she was asking her parents for $50 to pay for car repairs. Seriously, if you can't afford $50 for a car repair, how does she expect to raise a child? She needs to grow up and stop trying to imitate other people's lives. We all know the only reason she wants kids now is because her only brother and I have a daughter and a son on the way, and she sees the attention people give. Sigh.

EDIT: And the leeching off of others things is EXACTLY like my SIL. She owes my in-laws THOUSANDS. She's tried many times to get money from my husband and I. When we say no, that doesn't mean 'no' to her, and she keeps pestering. This is why I'm glad she's cut me out of her life. She won't even call here anymore. Yay!

Life's too quick to continuously act like an grownup. You gotta allow free typically and act like a child. Kids are continuously having amusing, with no care on this planet! On the turn part, youngsters is not going to check out to behave like adults. There's lots of time for that later.

Many adults still act like children because they have never been expected to act like adults. They have parents and family members that enable them to be irresponsible well into their 20s, 30s, 40s and beyond. If you SIL can make it leeching off of people and they let her do it, in her mind why should she change?