Why was Potifar's wife attracted to Yosef? Obvious reasons include the fact that Yosef was beautiful, and that through his success running the household he must have demonstrated intelligence and people skills.
There is one more possible reason which is often overlooked. Sefer Breishit describes Potifar as a “saris” (37:36,39:1). Literally, this word means “eunuch”. Now, it could be that since royal servants were often castrated, the term “saris” came to be used even for those who weren't. But it's also possible that Potifar was in fact a eunuch, and he married his wife due to social expectations , not out of sexual desire.
If so, then it's perfectly understandable why Potifar's wife would be unsatisfied with her husband and looking for someone to have an affair with.