Every thought that crosses our mind has the potential to manifest itself. However, its manifestation depends on how strongly we conceive that thought. Let's get it like this- our mind has deep grooves- deeper than what we think of. These grooves are a kind of container for our thoughts. Any concept or impression that we strongly believe in is stored in these grooves.
Every time we think about the concept or impression that we truly believe in, it just gets stronger, nudging the Universe to convert it into a tangible reality. Universe has nothing to do with our likes and dislikes (and ouch, that hurts!). But that is the way it is. Your thought may be good or bad- each time you set the thought in motion, the Universe starts acting on it without really differentiating between good and bad.
The universe is receptive. Just like wet earth is receptive to all kinds of seeds (be that of mango or melon), Universe is receptive to the energy that we are sending out. They say stars weave magic. Don't they? They do - but the real source of the culmination of the magic is you! Stars are just aligning their energies as per your thought patterns.
When we speak of thoughts and their manifestation, we essentially mean 'energy'. Energy vibrates with its own frequency. In science, energy is the 'ability to do work' - it's a force that can cause the displacement of an object. Simply put- here, the 'object' is your thought and energy will set the thought in motion (literally displace it) and send its vibrations out into the Universe. Karma essentially is based on this design- Universe will puke out what you've fed it with. It's the Law of Attraction- you get what you give.
The Law of Attraction and Astrology have a profound connection. Man is born under any of the twelve Sun Signs.
Whichever sign he is born under, he carries the impression of that sign. And that's why the ferocious Leos are so much different than the mushy Pisceans! These Sun Signs determine our vibrations and govern our thoughts and feelings.
In her book, Rhonda Byrne says, 'You get to choose what you want, but you must get clear about what you want. This is your work'. Sun signs can help us understand what we truly want- at least, they hint at what affirmations we can work on!