Is it reasonable for a patient to ask their doctor for an explanation of why they aren't concerned about an abnormal test result, or why they disagree with a diagnosis made by another physician? i always feel like a difficult patient when i do so?

It's not unreasonable at all. Ask them.

For as long as able to listen and understand OK it is highly recommended to ask questions within reason. Confronting a doc with another one’s diagnosis may not be productive as it is not likely to change the current’s doc opinion. That was the reason for “a second opinion”. It is now your turn to stick with the doc that you feel is right and you trust.