Girls, why do you get jealous of a"just friend" guy romantic interests, dating prospects or taste in women?

First, a little disclaimer: I DO BELIEVE THAT MEN AND WOMEN CAN BE FRIENDS, AND HAVE A BEAUTIFUL, PURE AND HONEST FRIENDSHIP.

But here is a bit more detail about the question.

This happens to me quite a lot, and i don't particularly mind it becauseI've had beautiful friendships coming out of it, but i would still like to grasp what is behind such behavior?

Le'ts say a girl and I decide to be just friends, why does the girl, in that case, becomes jealous and possessive? Why does she feel the need to compete or criticize every girl i look at or my type of girl? I had remarks, well you wouldn't look at her if she wasn't' like this, she has no class, no style, etc..
When i say; oh this girl has a nice style, i like her outfit, why doe the friend says: "well you know, i wore this outfit yesterday... just so you know"

And now to the last part of the question.

Why don't you, strangely, have no single friends, that you can introduce me to? Girls often say to me, that they either have no one and when they do the girls are either in a long term relationship, or (and they know it) not to my taste at all.
I also had one, that invited me to a dinner party at her place. When i asked if i could bring a date along, she became so nosy asking me loads of questions about the girl, and when i said it was my best friend, she hadn't met she chilled. But when i said, that my best friend couldn't make it, she was so happy and said it actually works better for me and made me sat down right next to her at the dinner table and didn't leave my side.

If any girl would be so kind as to shed a tiny bit of light on this matter, it would be greatly appreciated.

Thank you all.
Girls, why do you get jealous of a"just friend" guy romantic interests, dating prospects or taste in women?
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