Ah, I think I see some of your problem. No, you cannot walk up to strangers, especially if you are not of their gender, and just start talking to them; they don't know you at all, can't "trust" you because they don't know you or anything about you; you could be chatting them up to kidnap, rape, and murder them for all they know, they can't make a determination without any information and the unknown is scary.

They may feel they are being spied upon if you tell them something personal, that you think they are pretty; they did not ask for your input and comment about their looks and, because they don't know you, think it an odd thing for you to say to them first, so start thinking you are odd.

Interactions between people, especially people of opposite genders, have to build up gradually; you don't just say, "Hi, want to jump in bed with me and have sex?" and "You're pretty" is up there in the middle somewhere, a personal comment. The other person may/may not agree with you but there's no basis underneath to have that conversation yet because it was an opener, has no where to "sit".

The "boring" little comments form a base to build on, give a little understanding of the other person and what they are interested in and like so you can make internal comparisons and check to see if you are interested in further, more personal conversation.

"You're pretty" has nowhere to go, stops any hope of conversation because it is both superficial (I have brown eyes, too, so what?) and too personal; your judgment about my looks. No matter what I think or say, you don't appear to care (I can't "disagree" with you because that means I would have to negate myself and I can't agree with you because it's "my" looks and I get to decide how I look and here you are trying to tell me how I should think/feel about myself). I don't know you and yet you appear to have taken no time to think about how I think or feel, are busy telling me about how you think/feel about me, before you even know anything about me. It can't work. The "polite" response would be "Thank you" and then the woman scurries away, having learned you are out there with yourself and not thinking about her for herself at all.

If nothing else is going on, "you're pretty" is a non sequitur, doesn't come before or after anything. It is like the joke, "Do you walk to school or carry your lunch?" it does not make sense to the hearer for you to have said it to them. A woman might look around and see who you are talking to since it comes out of nowhere (your head/imagination) and is not based on anything that has been said or done; it's not warranted.

At the beginning of a relationship you talk about yourself but not in relation to the other person. That's why we ask questions of one another or talk about the weather. When we talk about the weather, everyone can relate and it doesn't feel like the subject is coming out of the other person's head.

One can say to a female checkout clerk at the grocery store, "I hate all this wind; it's too cold!" but not, "I like pretty women" because the checkout clerk was probably not thinking about women, pretty or otherwise and can't see what it relates to. One has to say, "I love all this wind; it makes the women's hair stream out around them and I like that look".

The clerk has seen women's hair in the wind and knows it is windy now and, because you say you like that look, she can infer you think such women pretty. She can reply, "Yes, I love the way my hair looks when it's windy" to let you know she's "with" you or she can say, "Ugh! You men may like women's hair in the wind but it's a pain in the butt for women!" to let you know how she feels about windy days and that she probably thinks you were insensitive and not thinking about real women, but just about what you like.

Mostly the reason you cannot be direct is because what you are thinking and feeling is not what the other person is thinking and feeling and one cannot "converse" unless one lays groundwork for what one is thinking/feeling and checks to make sure the other person understands and that one understands what the other person is thinking/feeling.

The trick in a conversation is to respond to the other person, not to one's own head or feelings. One uses one's head and feelings to make that response but "You're pretty" is basically talking to yourself; it's about the other person and they're the one in charge of themselves. Even, "I think you are pretty" doesn't work as an opener very often because it's too general, the other person doesn't know what you consider "pretty" and so the response is just "Okay, thanks (for telling me about yourself and what you think)."