Negative language is stating something a person can do, but then immediately forbidding it with words like “don’t”, “can’t”, “stop”, etc. Negative language puts you (and others) in a negative frame of reference.
Positive language simply informs the listener to what they can do. Positive language is encouraging.
If you say “don’t forget…”, the listener is more likely to forget.
If you say “remember to…”, the listener is more likely to remember.
If you say “I can’t…”, you’re probably right.
If you say “How can I…”, you’re now in the mindset of getting over obstacles.