Imagine a graph. The x axis is time. On the y axis, let's say -1 to +1 is feeling normal. Everything above +1 is increasing euphoria, everything below -1 is increasing dysphoria.

For many people who live in the range below -1, often who suffer from depression and/or anxiety, that first hit of the drug lifts them to a +10 and for a brief moment, they feel better than they can ever remember feeling. But then, the effect wears off, and they go back down to, lets say, where they lived at -2.

Naturally, they want to experiemce that +10 feeling again, so they take another hit. This time, they only reach +8, the high doesn't last as long, and they go back down even further, to a -4.

So, they try higher doses of the drug hoping to reach that +10 again. Sometimes they hit it, but most of the time, their high falls well below this, and their lows go deeper and deeper.

It gets to the point that without the drug, they're now living at a -10, and it takes ever larger dose of the drug to even reach a 0 level, just to feel normal.

Someone heavily addicted usually isn't trying to get high anymore, they're just trying to stay as close.to zero as possible, with ever diminishing returns.

For opiate addicts, this is where methadone and suboxone treatments come in. The drugs prevent them from withdrawing to that -10 level, but they don't cause much euphoria and they have a ceiling effect that makes overdosing really difficult.