Why does my saliva have a sweet taste toit?

Mixture of things. Saliva is mostly water mixed with proteins, enzymes, electrolytes and other compounds. Each of us has a different recipe and your may have a sweeter side to it. It is possible that you have food residue in your mouth which may be what you are tasting.
I do not know. In certain instances, with a very high sugar level, the body will try to dilute the blood concentrations by releasing it into the urine, and will also make a person thirsty (in an attempt to dilute the blood). The saliva being "sweet" may warrant an evaluation, to at the least check for diabetes. Good luck.