system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is raising our hear rate from exercise considered healthy while stimulants are considered bad for the heart?
gpt
The heart rate increase from exercise is self-limiting - e.g. if you over-exert you generally get tired and slow down.  Your exercise pushes to the limits, slowly extends them, but "tells you" when you need to back down.  

If you take stimulants you bypass that entire self-regulation system and can easily overdo it.