My partner (28M) is always suddenly sick when I’m sick (27F)

Ever since we moved in together (3 years ago) I’ve begun to notice a pattern; every time I’m unwell or injured and voice this, my partner is suddenly dealing with the same or worse.
At first, I didn’t take much notice, of course, viruses can spread, and maybe he’s just caught what I had, however when it started happening with headaches and my back injury I started to get suspicious.
Now its more frequent, today was the first time I told him I was feeling unwell in a long time, I had stopped for a while, not that I get sick often, but the few times I did last year I didn’t say anything to avoid that.
However today I felt so faint and without thinking just mentioned it as I sat down on the couch. He was busy playing cod. He stopped, paused the game, and straight away said that he felt sick too. He continued to say he felt as though his heart was racing so fast and clutched his chest. I asked him to slow his breathing and offered him a glass of water. His hands began to shake. At this point, I was getting a bit concerned and asked if he wanted me to call someone. He told me no, that he thinks it’s passing but feels a tingling sensation everywhere. At that point, I noticed he had his watch on that records heart rates, not sure how accurate, but when checking it, it was within a normal range. Then suddenly he said he was okay and continued playing his game. I was left honestly feeling so stunned lol.
He does this with arguments also, if I get upset over anything, he suddenly is 10x more upset about something or brings up an old argument. It’s as if he’s trying to compete with me.
Why do people do this?