why would someone tell a depressed perosn they are so happy

My ex and I started friends but he called me up to tell me he is so happy. He can probably tell them I’m depressed (experiencing depressed not from him). Why would he call me to tell me how happy he is? Why would anyone in general tell a depressed person they are so happy? The depressed person wants to be happy to and then is just frustrated that they can’t be happy too!