It takes strength to be gentle. Anyone can be rude, but it takes self-control, strength, and discipline to be patient and caring.
For example, do you get easily annoyed (and show it) when others aren’t understanding what you’re explaining? Be gentle with them. Try to explain it in another way. Whether that means using a different example (verbally) or using a different teaching method (they may learn better by reading or doing instead of listening), if you really want them to understand what you’re talking about, you need to adapt your teaching style to how they learn best. Regardless of whether your other methods work, at the very minimum you’ll become a better teacher by learning how to use different teaching methods.
Lastly, if you really want someone to learn something, you can be firm with them, but don’t be rude or make disparaging remarks. While that may motivate some people, it will make even more people “shut down.” Then, not only do you lose your chance at a teachable moment, but you also hurt their self-esteem and lose their respect (if only temporarily) in the process.