It has to do with reward/motivation. People are driven(motivation) to accomplish life/career goals to gain the reward. The actual inner workings are way more complicated than what I am about to say, but...We are generally trained to work and seek status, these are the external trigger but internally our brain is training each of us with dopamine. Now, I don't mean the brain releases dopamine and we feel good, generally its the other way around. It's more about dopamines effect on memory and motivation.
You work, you get a big paycheck, there is a dopamine release, you get a promotion, dopamine release, you buy a house, dopamine release, you end up in a high paying glamorous job, dopamine release. Anything you have been told to feel pride in or have been praised for basically.(now repeated rewards of the same variety will lead to diminished dopamine release, which encourages us to seek novel experiences, but thats a different story although its one facet of tolerance)
But when you are taking excessive amounts of dopamine producing drugs you've already got the "carrot" so to speak, the only thing you've been trained to do is hit the pipe and as far as your brain is concerned you are accomplishing all you would ever need to. But this re-wiring does take a little time, which is why people new to speed still get shit done, but the longer they take it the more they are satisfied by just getting high and sitting in garbage. Not only does it re-wire your reward circuitry, but the amount of dopamine release obtained from accomplishing a goal or doing something to be proud of is so minuscule in comparison to the amount obtained from drugs that it hardly even registers.
This is largely how addiction occurs in the first place, you've become wired to seek out the drug in lieu of any other rewarding behavior.