It has to do with looks. Not personality.
It has to do with personality though. A while ago, I read a study* on infidelity, in which they asked men and women in long-term relationships about their dissatisfactions and reasons for cheating. For men, the #1 reason was that they'd lost physical attraction for their spouse. For women, that their spouse was not meeting their emotional needs. This was reflected in that the women emotionally cheated to a much higher degree than the men. To me, this suggests that women - to a greater degree than men - tend to look beyond the physical aspects of a partner over time.
Thing is, by going in with the wrong attitude you're not giving the woman a reason to look beyond.
(*which I of course can't find now that I finally have a reason to bring it up smh)
Today I saw a couple and it always seems that you need to be a special kind of guy to make a girl genuinely laugh. Felt like crying, to be honest.
All it really takes is a shared sense of humour. So looking at couples who likely bonded over that isn't fair to yourself.
To OP: it's a perfectly valid feeling. We're ultimately social beings. Society, religion, and even family puts pressure on us to find a partner, and it can while single make you feel like a failure for no good reason. I'm not sure if you feel like that, but I do sometimes.