there are several schools of thought:
1. They are unique. Men don't have them, and they help identify a woman as a woman.
2. They indicate that a woman has reached maturity, the development of breasts is one step in puberty. They show that the woman is sexually mature enough to procreate.
3. They can display the health of the female. Large healthy breasts can be a sign that the woman is capable of having, and raising healthy children. Though a woman with small breasts can still produce milk, large breasts are seen as a sign of health and vitality.
There are probably several other reasons. These are all evolutionary and subconscious. Men don't realize that this is why they like breasts when they like breasts. But because these signs help us thrive as a species, they have become sexy.
They are seen as sexy because thats what men look for in a mate, so that they can produce healthy children together.
Another reason is that there is a bit of mystery to them.
Girls hide them, so Guys want to see them. Way back when, it was sexy to see a girls ankle because women covered up more than they do now. Even when at the beach. And showing your legs has been sexy, showing the midriff was sexy, showing cleavage has been sexy, etc... but for the most part, breasts have remained hidden away.