This is something that I have always struggled to understand and would really appreciate someone explaining...

Why exactly does the tax policy encourage us to hold assets subject to capital gains for a year or longer? I'm not asking "how" the tax policy encourages us (obviously the benefit of a more favorable tax rate), but more so what is the government's interest in the matter? Does day trading have some type of detrimental economic impact that holding a stock for a year or more before selling doesn't?