system
Given the following post, respond as if you're commenting on Reddit:
human
[MCU/any relevant Marvel universe] On Pym Particles and mental stability (mild movie spoilers inside)

Hank Pym says that he cannot be the one to use the suit due to it having taken a toll on him over the years.  In addition, Darren Cross is shown to be a bit nuts as he works on perfecting the Yellowjacket suit.  This shows that prolonged exposure to Pym particles can cause issues with psychosis.  However, nobody has a problem with allowing Scott to wear the suit.  Is he truly "expendable" as he himself says to Hope, or is there something about him that lets him use the particles safely?
gpt
Having read a good deal of Marvel comics and seen Ant-Man, here goes.

1) "The Toll" is likely figurative. He can't put on the suit because the last time he wore it, his wife died. Bringing back that kind of emotional pain would cloud his judgement, and jeopardize the mission. On top of that, Pym is far past his physical prime, so he would be at a great disadvantage if forced to fight opponents of the same size (either normal people while he's normal sized, or anyone shrunk down via Yellowjacket suit).

2) Darren Cross was likely already paranoid, obsessive, and jealous long before he began working on replicating Pym Particles.

3) Pym said that exposure to Pym Particles while unprotected was dangerous, specifically the head. All of Cross's failed experiments were conducted on unprotected subjects. It is possible that Cross was exposed to some amount of Pym Particles, inducing psychosis.

4) Pym chose Scott because he was a very capable thief, adapted to new situations fast, and was a decent man. Pym couldn't bring himself to risk his daughter's life the way Janet did, but also couldn't hand the suit off to someone who he suspected might just take the technology for themselves. Pym obviously didn't want Scott to die.

Comics: In the comics, Hank Pym was both Ant-Man and Yellowjacket. He developed split personalities as a result of the stress of his work. The psychosis in the movie, as well as Pym's comment that he "saw himself in Cross", are likely references to this.