Because you inch closer and closer to where you yourself are simply running out of time to enjoy with your very own senses anything your BTC can buy. And thus - as I see it - the sensorical value someone gets by spending some of their BTC will at some point in time turn almost every hodler into a net seller. Some people are lucky enough to have enough cash flow to enable a satisfying lifestyle _as well as_ buy/hodl BTC without _ever_ selling any. But I'm pretty sure these people make up a small fraction of all hodlers. All the others will - IMO - turn from buyers into sellers at some point in time.