1. You don't take the initiative to say hi to them when you see them or when you're at their house.
Why a person won't do this is just plain weird and impolite. It's like they're nothing to you, when they should mean something since they are your guy's parents. If you snub them, you'll give them a pretty bad impression of you. You're the girl who doesn't know her manners. What's to like there?
2. You don't even try to chat with them when they're trying to converse with you.
It's one thing to not take the initiative and reach out to them, and another thing to entirely dismiss their efforts to get to know you. You may not be intending to give one-word answers. You may just be really shy. Guess what: Your shyness is coming off as impoliteness. You're making it seem like you don't want to talk to them at all or that they bore you. Sometimes you have to get over that shyness and just CONNECT with people. That's the well-mannered thing to do.
3. You don't say thank you to them after they have you over for dinner.
This is another well-mannered thing to do. They fed you with what they have, entertained you, and did what they could to make you feel at home. The least you can do is to thank them for it. Don't be a bitch. For plus points, take the dishes to the sink and offer to wash them.
4. You dress a little too promiscuously.
Okay, your clothes may not have anything to do with your chacter and personality. But when you're in revealing clothes for a Sunday family brunch with them, you're going to give an unpleasant impression. They might not think you're slutty or flirty (hopefully they're not narrow-minded), but they'll think for sure that you don't know your dress codes. And someone who doesn't dress appropriately is bound to make a bad impression on anybody.
5. You get your PDA on in front of them.
Yes, kissing is natural for us when we're in love. But heavy touching and making out in front of them is just out of place. Wait 'til you and your guy are alone in the room, or go get one.
6. You talk badly about other people or you gossip in front of his parents.
The thing about gossiping with or in front of people you're not close to is that you let them know that you have no filter and that you're out to ruin someone. You let his parents think that you could be talking smack about them too, and who is comfortable with—who can trust—someone like that? Remember that talking shit about someone says more about you than the person you're hating on.
7. You're inconsiderate of your guy.
If you're the type to yell at your guy or demand things of him (like time or money or gifts) when he's got his hands full, his parents will not like you, especially when they see how much you're tormenting him. They want someone kind, loving, and understanding for him. Life already demands too much from us.
8. They feel like you have no ambition in life.
And they have that impression when you say you don't really know what you want to do with your life, or worse, when you're unemployed and you're doing nothing productive with your time—that is, you're not learning anything new hence you're not growing. Parents want their future grandchildren well provided for. That's only normal, if not natural. So when you can't even seem to look after yourself or care about your future, what about of your future children? Okay, you might not want a family, but still, your lack of ambition and drive is going to weigh your guy down when you guys decide to tie the knot and settle.
9. They see you're too much of a big spender.
This is much, much worse when they think/know that you're not earning a lot. They wouldn't want their son to marry someone who doesn't know how to stop herself from splurging on unnecessary material things, someone who's not smart with money, or simply has no self-control. Or worse, for their son to end up with someone who'll end up using or taking all his money. Being able to handle money well, to save, to invest, shows that you're a thinking, responsible adult who can be on her own. (Meaning you're with a guy because you love him, not his things.)
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