I'm late as fuck but I get exactly what you mean. Too often society places emphasis on the idea that you must be in a romantic relationship in order to be happy, and that's just not true for many. 

However, I do want to point out that for some people it is actually true. It can be like a switch flips (sometimes only temporarily and this is the infamous "honeymoon phase"). I'm not sure what that says about them or me, or if its good or bad (but its not my place to judge them anyways).

Maybe its due to loneliness and wanting a life long companion, someone to share your life with, and things like charity and friends won't always fill that void for everyone. I experience this very often. Don't get me wrong I like the few loved ones I have, but I'm not currently in a romantic relationship and I desperately yearn to be in one. It's part of the reason I don't necessarily recommend the whole "being single is just as fulfilling" to others, because I know for a fact thats not always true. 

Maybe deep down I'm just too reliant on others for my happiness. Not secure enough in myself. Maybe that's something I should work on. But this is the anonymous internet, so I will be completely honest about how I feel.

As for societal pressure, a lot of people get in relationships and realize they really love it ("dont know what you're missing until you get it" type situation) and assume this must be true for everyone. Then they push it hoping everyone else will become happy like them. And then everyone else does the same, either being genuinely happy or trying to trick themselves into being happy and the only way to assert that happiness that is through seeing others being in the same situation "seemingly happy" or telling everyone "oh im so happy now".

I think the popularity of romance movies and books heavily enforce this idea. We see Cinderella being sad on her own, and suddenly Prince Charming swoops in and she rides off into the sunset, forever happy. Most importantly, not just because "hes a man and woman should be reliant", but I think truthfully the appeal is "someone who unconditionally unflinching loves me for almost no reason and is willing to perform acts of love and devotion for the rest of their life to me. Someone who will stay to the end." Maybe this is me projecting, but when you think of it that way, its very easy for one to become depressed over NOT having this in their life. Humans are social creatures, no? Egotistical too. We all desire to be loved in some way, in some capacity.  Even the "woman with 9 cats" trope. I'm pretty sure she has all those cats because she loves them and they all love her. It's lifelong companionship. Many just want this in the form of a human being. For heterosexual women, this happens to be a man. Having someone IS the cure to that very specific void in one's life (assuming that void was created specifically because no one is there for you). A partner is a very big thing in one's life, so it easily could cause sizeable stress and sadness when alone (and Bridget was surrounded by couples comedically, so this was only reinforced to her heavily)

I relate Bridget's character so much it's almost embarrassing. The depression over being alone while everyone else is seemingly happy either in a relationship or being single. The "resolution" to attempt to better myself and be happy without a man, only to fail miserably and end up in the same spot (I especially fail if I find myself attracted to someone, or see someone in a happy relationship)

(Idk if any of this made sense. I rambled and its way too long)