Why pain in left testicle?

Some reasons...: True testicle pain means a trip to the doctor right away because the testicle's blood supply can be twisted off (called torsion). When that happens, the testicle can die. Other causes of testicle pain include trauma, infection, twisting of an appendix of a testicle (a small lump next to a testicle), etc... Since one usually cannot tell if his testicle is twisted, an exam by the doctor is needed.