What is it about breasts?
When scientist Ivan Pavlov discovered that he could cause his dog to salivate with the ring of a bell after the dog learned to associate the sound with food, he set in motion extensive research on what is known as classical conditioning. Twisting this concept, it is possible that your partner has been conditioned to associate breasts with sexual pleasure. So now, instead of the dog, it's your partner, and instead of food, it's sex. If replacing the bell stimulus with the sight, thought, or touch of your breasts, your partner responds by becoming aroused or just generally "smiley-eyed." Considering how breasts are constantly associated with sex and equated with attractiveness in Western culture, it's no wonder why your partner and many other men and women, gay and straight alike, are conditioned to respond in this way or feed into this cultural expectation.
Because of this association, unlike in other parts of the world, the general expectation in Western culture is that women keep their breasts covered unless they are engaging in sexual activity. This leads to another potential explanation for your partner's boob-session, which revolves around the idea of the "forbidden fruit." For many, when an object becomes scarce, taboo, or difficult to obtain, it becomes that much more desirable. Support for this hypothesis lies in the fact that until the introduction of Western influence, the sexual connotation of breasts was not found in Africa or many parts of the East. Furthermore, there is no evidence that other mammals (even our closest primate relatives) share the same sexual interest in breasts as humans.
Since breast size has no utilitarian value (meaning, bigger breasts don't produce more milk), another possible evolutionary explanation is that "pendulous" breasts have developed as a sign of fertility to attract mates. Amongst our closest animal relatives, female genitalia are often eye catching whether in color or size and may change depending on the female's level of fertility as a signal to potential mates. Once hominids began walking upright and the buttocks and genitalia were no longer easily visible, average breast size may have increased to make up for this. Recent studies even show that women with larger breasts tend to have higher levels of reproductive hormones. Subconsciously, your partner may be acting on evolutionary instincts.
Whether he's been classically conditioned to ogle your breasts, or he has developed a desire for your melons because Western society deems them "forbidden," or Darwin had it right when he espoused his "survival of the fittest" theory, your guy is in the company of many other men and women. Keeping abreast of the possible explanations for his reaction may still have you scratching your head, but if you enjoy the chest attention, milk it for all it's worth.
In a society and culture where we're always looking for the next breast thing, hopefully this response isn't a bust!