I can't think of a reason to be happy alone.
You can't think of ANY reason to be happy? If this is actually true, I think you are pretty clearly experiencing symptoms of depression, probably for reasons that originated before being alone was even a concept that you really understood. Depression is a treatable sickness although you may need to seek professional help to do so depending on the severity and duration of your symptoms.
If you are exaggerating a little, as I suspect you probably are, then I would suggest to just keep doing things you enjoy and try not to give yourself time to give a fuck about being alone until you actually don't. This is what generally worked for me anyways. Of course, you have to be a little smart about doing things that you enjoy as well, as some things such as doing copious amounts of certain substances and getting fucked up all the time, while you may well enjoy this (as I do, and I'm sure many of us here do also) the net effect if you do this will be that you will reduce your overall level of happiness and your depression (or "almost like depression" symptoms, or however you want to phrase it) will not improve. It's cliche but really just focus on positive things like exercise, keeping intellectually stimulated, and just learning to want and enjoy doing things that will make you a better person.
On that note also, it's important to remember that
love is a drug that all human beings are genetically hardwired to be addicted to, to some extent, and for better or worse. As we all know, we can't always do the drugs we want to do all the time, and when we can't do one particular drug we need to learn to just accept this for the moment as dwelling on it and pining for it has no value to either yourself or anyone else. I do think that to some extent, because of this hardwired addiction, the old cliche that you need to love yourself before you can love anyone else, yadda yadda, is probably not entirely true, rather it is a
learned discipline of spirit to be comfortable with yourself in the absence of what is arguably an innate human desire.
I deliberately did not say "human need" because I do not actually agree that love, objectively, is a need. You cannot die from lack of love, at least not directly, in the same way you would die from lack of food or water. I think this is often largely a problem with how you frame your own life to yourself. Even describing yourself as "alone", while it may be true, is an unnecessarily negative perspective. At the moment you are a lone wolf,
and if love is the only thing you are lacking, you already have everything you need!
But, having said that, I do think that learning how to be comfortable alone is a necessary discipline if you want to be able to cope with NOT being alone ever. If you are NOT able to be comfortable being alone, then when you have the opportunity to not be alone anymore, you will very likely not be capable of having a healthy and mature relationship, and will be far more likely to enter into somewhat dysfunctional and codependent relationships where one or both of you is overly reliant on the emotional support of the other... so for the benefit of your future not-alone self, and any future partners you may have, I suggest digging deep within yourself to find this discipline of spirit and be comfortable with solitude. Again, it is OK to seek professional help if you need it - I wish you the best of luck in your journey!