I wonder how these people feel qualified to tell me that my concerns are groundless, while not knowing what my concerns are. It resembles calling the fire department to report seeing smoke coming from a building across the street, and being met with, "I don't see any."
And I ask these people entirely hypothetically, what would be a good concern? What would be a valid concern? Imagine something, no matter how ridiculous, that would cause you to question the validity of the Church's claims.
I can think of a whole lot of stupid reasons to leave the LDS Church, or any church for that matter. Some leave because they get bored. Some feel they don't have time. Others leave because they were offended by something another member said. Some just really want to drink coffee, or watch rated "R" movies, or sleep in on Sundays. Some claim to believe in God, but only up to the point where he tells them how to live their lives. I think these reasons to leave are all mere excuses, and not enough to warrant such a decision. If the LDS Church's claims are true, then we are obligated to follow its doctrine. The only reason one can reasonably leave a church is if its claims are false, its foundation a fraud.
But on the other hand, it seems to me that active members think there could be no good reason to leave the Church. Or perhaps they don't want to know that there really might be good reasons.
On my mission, I remember a particular person upon whose door we knocked. He was very polite and listened to our introduction of the Book of Mormon, but when we asked if he would like to find out if it were true, his answer puzzled me greatly. He said, "Oh no. I don't want to know if it's true. If it's true, then I have to do something about it!"
So maybe ignorance is bliss for some people. They are comfortable where they are, and learning something new could force them to see that their view and interpretation of existence is flawed. Maybe some of those who don't want to know exactly why I left the Church are afraid that I do have very good reasons for doing so. Maybe they don't want to know that my criticisms of Joseph Smith and the Book of Mormon might be true, because then they might have to do something about it.