Ok here's a thought. Since the 1920s the S&P has delivered an annualised return of around 9%. Some years it hasn't but on average it's 9% plus. So why would you sell it ever?

Well I guess if you brought it cheap, and now it's expensive, one reason to sell some would be because it's overweight in your portfolio and maybe you have something that "mite" perform better. 

But that's a big mite. $Vanguard S&P 500 ETF(VOO)$ Is at record highs, and continues to bounce. My perspective is that I'm not overweight, so I have no compelling need to sell. If it drops significantly I would just buy more, funds permitting. If you have the luxury of time in the market then there is no need to sell anything that is good. Investing in stocks is a long term play for me. I don't listen to the Wall Street noisemakers, they want you to trade so they can make money at your expense. They try to sow fear but they really have no clue. If a stock is worth $100, but you brought it for $50 should you sell it? Well it mite be worth $200 a year from now, or it mite drop back to $50. So you could do exceedingly well, or be at break even. But if you have done your DD you hold the conviction that it's a great stock. At $100 it mite only be giving a 2.5% divided return, but you brought it for $50, so your return is 5%. Why take a short term profit? You planted a seed and it's grown into an apple tree. Cut down the tree and sell it for firewood and you get a nice short term return, but there are no more apples going forward. 

Personally as an emotional investor I think of my portfolio as an orchard. I will only cut down a tree in that orchard if is no longer going to produce, and plant a new seed. But if I get a great offer to sell a tree that is and will continue to produce, I am not cutting it down