system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is a slow resting heart rate good, when it is also healthy for your heart to beat fast during exercise?

EDIT: So several answers have come in, but I'm not sure my question is being understood.  I'm asking, if it's healthy for your heart to beat fast during exercise, why isn't it healthy for it to beat fast(er) during rest?  Wouldn't a higher resting heart rate almost be like a mini "workout?" Making it stronger?
gpt
its about strength and efficiency. 

Your question is quite valid... Why can't we just do Cocaine or drink caffeine to increase our heart rate for "exercise"? Or just be panicked (increasing our heart rate) to get a good workout? 

It takes a little knowledge to come to a good conclusion. Ahem.

There's an upper and lower limit to your heartrate. Everyone's different depending on age and health. We've established some "normal" ranges. About +- 70-85 BPM at rest is normal; then the top number is dependent on some factors that you can look up on your own.

Anywho, the heart is a muscle (duh) right, and it's activated and regulated by the bodys need for it's service (pumping blood). During stress it increases it's rhythm for both psychological (anticipation), and physical (sustaining, survival) need. More on this in a min. 

So depending on how your health is, the body prefers a sorta calm state; (feelsgoodman.exe), your average resting heart rate lies in this realm. Anything above this indicates *some* sort of stress on the body. There's healthy stress and unhealthy stress. Unhealthy stress includes: unnecessary psychological anxiety/fear, drug use, sickness. While good heart stresses are things that lead to better states like: exercise to "look better" or strength to protect yourself, or even light activity like getting some food from the store (food endorphins bruh). Regardless of what you do, your body is reacting in harmony with what will sustain you.

So it's like just because your HR is high or low doesn't mean it's good for what you're after. Like a higher BPM is normal in sick people but your body is battling something which is good but if you lay around sick for months your muscles waste away because of lack of use . The body is always changing according to automatic (unconscious) and consious need.

Understanding the hearts basic role then leads  us to better answer your questions. You **gain** something from the **right** high heart stresses which are reflected in your heart rate. Consistent exercise (along with healthy eating and habit) nets gains both physically and mentally "look good feel good" (dopamine bruh). The bodies equal reaction to being well all around is a lower HR at rest when calm. 

There's plenty more to look up about heart health that's awesome. You can get a baseline of your health with your sleep HR vs resting HR vs exercise HR vs rebound to resting HR after exercise, this is almost a sure way to indicate overall health.

Hope this was helpful.