In the old days it was socially correct for one to help guests or fellow diners first when a dish was served, especially if you were playing host in the table. Some would do so throughout the course of the meal. Fellow diners would also usually reciprocate. All these were supposed to be traits of good upbringing. Using your own pair of chopsticks to serve others was also not a big issue, even though they had been coated with your saliva many times over!
But friends, this is NO-NO in the present time, unless the person you want to help is a lady, or an elderly, or a young child! Even then, please DO NOT use your chopsticks, or fork, or spoon. Ask the restaurant helper for a common pair of chopsticks or a fork or a spoon if none is provided. It is still quite common to see Chinese eateries and restaurants the world over to overlook this need even today. (Yet we say we are the most cultured people on this planet!)
First of all, many people are quite health conscious or discerning about their food now. What is nice to you may not be great to your guests or fellow dinners. Second, you spoil people’s appetite with your “peasant” habit of wielding chopsticks left, right and centre.
Another peasant habit one commonly sees is this: dumping the bones and fats on the table. Thoughtful restaurants usually provide a small plate beside the main plate/bowl for the purpose. But if it is not available, do leave them at one corner of your plate. When you are done with the meal, don’t leave your chopsticks and cutlery all over the world. Place them neatly together - parallel to one another - on your plate.
Many of us Chinese still have a great deal to lean about table etiquette!