You should look inside - we cannot possibly know the real reason.
But in general, if you're traveling solo
>during the day most locals work, or are traveling with a purpose, and in general are busy with something
>meanwhile you just go from place to place without any particular goal, but rather to see some places
I think this may be the origin of your sad feel. That's probably also why you're feeling better in the evening, since everyone around is also chilling, eating and drinking, and looking to have fun.
My theory is if you want to have a fulfilling and joyful solo trip, you must either
>have a purpose, fresh goals, or constant problems to solve
>make new friends and in general connect with more people on your way
I travel solo exclusively and to make it fun I don't book shit in advance, don't research too much, try to improvise and surprise myself. This creates small "problems" which are entertaining to solve,
if that makes sense.
I also meet new people all the time - I make friends with airbnb hosts, couchsurfing crowd, people who sit next to me on a plane or a bus, or at an outdoor terrace.