To think in bitcoin is to make Bitcoin the primary unit we use to measure value. The modern world will pressure you to use other tools: fiat currency, instagram follows, gold, oil etc. But, since Bitcoin is the only object that is verifiably finite, it should be used as when measuring value.

If you were to use any other asset/tool/object, the value you record today would not be the same tomorrow. Nominally, yes, but not relatively. The total supply of the fiat currency you used could have expanded, more gold could have been mined, and a large supply of crude oil could have been discovered.

Supply growth is a distortion.

By thinking in Bitcoin you also can clearly see when something is overpriced. Is Apple worth almost twice as much as the Bitcoin network? Of course not. If you come from the perspective that there can only be 21 Million units, then you can measure current Bitcoin exchange rate for that thing and extrapolate from the premise that "Bitcoin will reprice everything" and see that just about everything is overpriced in Bitcoin.

Another advantage to thinking in Bitcoin is how easy it is to make economic calculation and analysis. When looking at activities you are considering taking or even past actions, you ask yourself "Did this action lead to more or less Bitcoin?" or "Will this action lead to more or less Bitcoin?" and based on that answer you will know if you have or will make a good economic decision.