system
Given the following post, respond as if you're commenting on Reddit:
human
CMV: open border agreement between the United States and Canada would benefit both nation's citizens

Have something similar to what the EU enjoys - no artificial limitations or arbitrary requirements from crossing between the United States and Canada. You show up at a check point, show some official documentation to prove your ID (such as passport), they run a background check for criminal history, if nothing serious comes up, you're free to pass. Open borders does NOT have to mean no borders and zero checks. The definition I'm using is similar to the EU's, which could include checks and expelling anyone who poses a threat to natives. 

The economy for both countries would improve, if people have the freedom to take their skills and expertise to Canada or the US. The US pays more, but Canada provides better quality of life, both countries offer something different depending on priorities. Government welfare in both countries increase with greater productivity. You shouldn't worry too much about brain drain, people do return, and they bring back experiences and contacts to improve the industries in their native country. Both countries largely speaks English, get along fine culturally, compatible and comparable policing and crime rates, so there shouldn't be too much tension. 

I like to think Mexico would eventually join too, but baby steps first, Canada & USA sounds like an obvious win-win.
gpt
The issue is that the U.S. citizens entering Canada would leech the health care system and not nessecerily plant roots within the country.

This would then turn Canada into some kind of proxy health care state and would instead just encourage people to live in U.S. border cities instead of in Canada proper.