Does one have to wear his Tzitizis on the flesh of his body, or can one wear his Tzitzis over his tee-shirt. If not, why?
There is no problem in wearing the tzitzis over one’s undershirt or tee-shirt, and this is common practice for most observant Jews. In fact, many are particular to wear the tzitzis over an undershirt, so that the garment will not become sweaty and sullied, and not be in direct contact with the body.
Sources: See Kaf Hachaim 8:3, who writes, based on kabbalistic principles, that the tzitzis garment should not be worn on the flesh. This is also stated by Rav Chaim Kanievsky (Techeiles Ve’argaman 5), and by Teshuvos Vehanhagos (1:25), for fear that the garment would be considered an “undershirt,” which might be exempt from the obligation of tzitzis.