In years like 2017, it has been easy to get a bit lax about your portfolio. If you're busy, the fallback has likely been to peer at your enlarged balance periodically, say "Yay for me!" and leave well enough alone.
That sort of laissez-faire approach has been rewarded thus far this year, especially for investors with sizable equity stakes. Through the end of November, the S&P 500 has returned more than 20%, and Morningstar's Large Growth Index has gained nearly 10 percentage points more than that.