I would like to clear something up if there was any confusion over 9's emotion waves. Basically, his kind is built upon the principle of sharing everything. Each individual of 9's species is connected through a vast hive mind of sorts, each one is an individual but also highly dependent on the others for support. Both physical and mental. 

As such his species are broken up into casts and sub castes. 9 is a warrior type, but specifically he is a guardian which gives him a strong sense to protect almost anything smaller than him that isn't threatening. So the smaller castes, mostly hatchlings, depend heavily on 9 and his siblings for protection. In return, they help ease any emotions that may build up. 

Now for the important part, the reason 9 needs to release these emotions onto others is because if he didn't it would take control of his life entirely. Example, a moment of extreme happiness has happened and if 9 doesn't share that with others he will literally become happy. All the time. Literally nothing could cause him to become unhappy, but in this state he wouldn't be much help to anybody. The same goes for every major emotion. However if its a small thing, like a hug from a friend or a bump on the hoove than he doesn't need to share those emotions. 

Only when they become too much to handle on his own will 9 share his emotions with others.