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I have to say that though we may know how characters are evolved into what they look like today, even historians or archiologists probably would have no idea why ancient people choose this specific character to represent its meaning in the first place. We can only postulate, but we would never know the truth.
As for "星", it is composed of three "日" above and one "生" below. "日" is an object in the sky. If there is only one object, it means the sun. If there are many they are the stars. Actually I wonder why “晶” desn't represent the stars. But ancient people give "晶" another meaning. Three suns must be very bright, so "晶" means bright.
Back to "星", stars are very important to ancient Chinese. They observe stars and make prophecies. For example, the rising of a new star means a very important person was born. So stars are important to people's life. Actually, people say that every single star in the sky represents a person living on the earth. That's probably why they put "生" below. Stars and people are connected.
