It is the person themself who decides happiness, not the extraneous variable in the shape of a relationship. If a person is therefore a happy go lucky person then it does not matter if they are in a relationship or not, and vice versa.
Personally, I have been single for 3 and a half years after being widowed. I miss her with all my heart, but all the things I loved before and during our relationship, I still love dearly. So I guess I am happy.
Without sounding self-pitiful, I imagine I will be single, probably for the rest of my life. I have to plan to share my life with anybody. I do my own things, have my own friends and basically have absolutely no committment whatsoever. Why complicate issues? I am also incredibly critical and fussy when it comes to birds. Very few indeed will meet my criteria, and if they do they are probably already snapped up.
Besides, I live in the country with spiders the size of saucers a regular thing. No bit of skirt would dare enter my house, let alone my life.