1
It's clear that breasts are a particularly erotic area of the female body. Oddly enough, female humans are the only mammals whose breasts are enlarged even when she is not pregnant or breastfeeding. For men, they're beautiful and for women they're a source of pleasure if stimulated well. But more than that, men can't help but be attracted to breasts. There's a primitive relationship between men and female breasts, since they're the first source of food at birth. Breasts evoke in men that special bond that's established with their mother through breastfeeding, during the first months of life, a happy and safe time.
2
Like a wide, shapely hip with pronounced curves, breasts are also a symbol of fertility. Their growth marks the end of childhood and the beginning of early adolescence, a time when a woman is ready to reproduce. This association made by our ancestors is still very relevant.
3
While we've evolved significantly from our primate ancestors, we continue being animals. Therefore, when choosing a partner, we're unconsciously attracted to certain physical aspects with the ultimate goal of ensuring the survival of the species. This is one reason why men are attracted to women with big breasts. It's a symbol of fertility that they can't help but feel a great attraction to.
4
Stating the obvious, the fact that breasts play an active part in the sexual act inevitably makes them attractive and exciting for men. In addition to being constantly covered by clothes, emphasised by a good bra or revealing a tempting bit of cleavage, men's imagination carries them away, further stimulating their fascination with this part of a woman's body.
5
Of course, a woman's breasts are also a very pleasing and beautiful part her body. Arousal happens in the first moment, with visual stimulation. Hence, breasts are very pleasing to most men, with their pretty shape, proportional size or soft and delicate skin, all powerfully exciting elements for them.