There are two reasons for the taboo against incest.
One is increased risk of birth defects and genetic diseases, if two close relatives do choose to have children. However, this is commonly misunderstood and exaggerated, so let's talk about genetics for a second.
Most traits are caused by looking at two or more genes and expressing the most dominant one — there are exceptions, and it dos get much more complicated than this, but this is just a basic lesson. Diseases caused by faulty genes are usually recessive — meaning that someone with a more dominant gene can carry them but never get the disease or even know they have that risk factor.
Close relatives have much genetic material in common. Therefore, if one person is a carrier for a genetic defect, their nephew, sibling, first cousin, etc. Is far more likely to also be a carrier than a stranger.
If two people carry a recessive genetic defect of this most common type, their child has a 1 in 4 chance of getting the disease it causes. If only one person is a carrier, there is no chance.
So, that is how incestuous relationships got a reputation for causing birth defects. However, thanks to modern medicine and genetic screening, this risk becomes less and less of a problem. The main reason for the continuing taboo and the modern laws against incest is actually much more disturbing:
Most incestuous relationships form out of abuse. Most incestuous relationships are in fact not consensual, or form by brainwashing (e.g., older sibling conditioning a younger sibling, or parent conditioning a child, to believe that a relationship that treats the younger as merely a sex object is normal and healthy) or other forms of manipulation. The laws exist to protect people from this kind of predation within the family.
Really, there is nothing inherently wrong with finding a relative attractive. Things are only wrong if they hurt someone. However, the taboo that exists against it does exist for a very good reason — even if many people only blindly follow along with it and get squicked just because they think they're supposed to be squicked (which I believe is something that shouldn't happen — critical thinking is an important skill — but that's another story for another day). Do know that if anything comes of your attraction and neither you nor your cousin are hurt by it, you are the exception and not the rule.
