system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5:Why is it bad to refill the same plastic bottle?
gpt
It is not. It is a psychological marketing trick to increase the sale of bottles. 

Plastics can leech into the contents (e.g. water) of the bottle if it gets damaged either physically or chemically. But a standard polyethylene bottle will last a very long time when if treated well. 

Companies have been/tried to have been sued for people using incorrect types of plastic bottles to store thing that they were not originally intended for. Like paint or solvents in old drinks bottles, and this can cause some bottles to degrade faster. So for liability reasons companies also say it's bad, but it's just the standard idiot label. Similar to how a bag of peanuts has a warning that the bag contains nuts. 

Long story short, it's a marketing trick masquerading as a liability trick, in turn masquerading as a health issue. Be smart, refill your bottles, help stem the plastic tide!