The key: Don’t tell a person you ‘need’ something when you merely ‘want’ it.
Some people tend to use the word ‘need’ to persuade people to give them what they want. That’s unfair, and it can be counter-productive, because people don’t like being manipulated. They might resist you.
Furthermore, if you use the word ‘need’ you might end up persuading yourself that you do need it, and give it too much importance.
In short, get into the habit of using both words carefully, and gain a healthier perspective.