Help please!!!!!!!!!!!!! serious?

i have a lump on the top of my left testicle its soft/squishy has a dull ache when squeezed liek when youd squeeze your teste and its movable i went to the doctor about it he smiled and said its harmless heres my question if you cant tell if you have testicular cancer or not why did he seem like it was nothing? he said it was a part of puberty btw