"I've read science fiction stories about being immortal, and in those stories immortality gets really boring, really fast. I'm not interested enough in reality to be in it forever." I can't see where this perspective could come from other than mind-numbing ignorance/the unimaginable nature of really big things (like the number of languages on Earth, the amount of things we still don't know about physics or the fact that every person who is or ever will be is a new, interesting being to interact with.)

We should avoid generalizing from fictional evidence, but fiction can give us insight. I suspect that this sort of statement generally is not a person's true rejection, though. On the off chance that this does comprise some portion of the thinking behind a person's deathism, you could try recommending science fiction stories where being "immortal" doesn't get boring. An example: Peter F. Hamilton's series, the Commonwealth Saga and the Void Trilogy. The latter series takes place over a thousand years after the former, which takes place several hundred years in the future relative to the present day. Some characters who were born in the 21st century appear in all of the books, having lifespans over a millennium, and they are far from bored.

"I can't imagine being immortal. My idea about how my life will go is that I will watch my children grow old, but I will die before they do. My mind/human minds are meant to exist for longer than one generation." This fails to account for human minds being very, very flexible. The human mind as we know it now does eventually get tired of life (or at least tired of pain,) but this is not a testament to how minds are, any more than humans becoming distressed when they don't eat is a testament to it being natural to starve, become despondent and die.

It's a valid point that we would have to make some improvements to the human brain in order for longer lifespans to be enjoyable. Neuroplasticity decreases as we get older, and advanced age can bring Alzheimer's, dementia, and other such disorders. As with overpopulation, though, this is another problem to solve, not an absolute objection.