I do need some help!! I've got some question about english that I do not know where to clear it up.....pleas?

Why the sentence: Our only fear is losing you.
Why the ing there? Is it
because of the fact that it might be the subject??

why do they use 'sort of' and 'kind of' (sorta/kinda) ALL THE TIME?? an
example: We've sort of known each other for a long time. Or: you looked
kind of mad. Or: Are you afraid?...Sort of.

What's the meaning of yet at the begining of a sentece??

And the last one: why sometimes I've read a negative sentence writed as a question but it meant negative. An example: Don't you love in vain, cause love won't set you free.

Thank you very much for your help.