Because no one ever knows where the highest price will be and where the lowest price will be. Your purchases and sales are random points on the chart.
It can be random, but you determine when to sell and at what price, when ever there is a rally, we expect a price to be the break out point where the correction starts and it is not always easy to determine that resistance level, but one think i do not do is to buy at a high price and let go at a lower valuation, just be careful while you invest and that is the only sensible thing you can do.