Why brain can't get used to low dopamine levels? When you use dopamine increasing drugs (such as cocaine) for long time the brain down-regulates dopamine receptors.Then dopamine has less sites to bind, therefore not making the same effect as before, which decrease our happiness and mood.

On the other hand low dopamine levels can cause ADHD or depression.I wonder why our brains cannot do the reverse of it (up-regulate receptors) when our dopamine levels are low?
Look at it this way. When rain falls on a roof, the water runs into a gutter. Up to a point, that water can run down to the drainpipe. The gutter has caught all the water, and the system is in equilibrium.

In your first example, the problem is too much water. The gutter overflows. The drain is accepting the same amount of water, but the gutter can't hold all the incoming water.

In the other example, the problem is not enough water. The gutter is sending it all to the drain, and the drain is taking it all away. There's less water coming out of the system because there's less coming into the system. Even if you made the gutter larger, you'd have the same throughput.

To answer the actual question, those receptors can only process as much as they get. If there's not enough dopamine coming in, the receptors can't make more out of nothing. They can't multiply the effect, only dampen what's there.