Why do people overdose ?

There are many reasons. Some people take overdoses in an effort to commit suicide. People commit suicide for a whole range of reasons.

Some people take overdoses but do so accidentally. They may have forgotten they recently took a dose; they may not be certain what the dose should be; they may be confused. The latter can be for a whole host of reasons.

Others may do so not to commit suicide but as a call for help.

Some people are depressed and think killing themselves is the answer.

Some don't realize how much they have taken.

Some are angry and want to punish their family or someone else by killing themselves ("That will show them").

Some think they can take more without consequences ("I'm tough").

Many reasons

Opiates (Codeine, Morphine, Heroin, ...) act upon mu receptors. The brain regulates how many receptors are active at one time. If you give it too much. The brain will start deactivating receptors. That means you need a higher dose to get the same affect. Until the rest of your body cannot handle it and fails. The biggest is lung failure. Opiates decrease lung function. Too much and your lungs stop working.

If you stop taking it, the brain will start reactivating these receptors again. This is an issue for recovering addicts who relapse. They took X amount before so they will take X amount again. Not realizing that now there are more receptors activated then when they last abused it.

maybe they are falling for that, "more is better" thing, When more is not better, the right amount is best.