Why do I test positive to herpes type 2 and my partner to test negative?

Contagious but... Herpes is a contagious std which is at its most contagious when you have active lesions.When lesions are not present, the disease can still be contagious but less so. Things that can reduce the odds of spreading to your partner are avoiding sex with active lesions, using anti-viral suppressive meds daily (your doc can help with this) and condoms, but nothing is perfect. Try www.Herpes.Org for info.
Partner not infected. Your partner doesn't have it. Believe it or not, genital HSV2 is not transmitted all that easily, probably averages once for ever 1,000 unprotected vaginal sex events (higher risk if a noticeable outbreak is present). So many couples go for years or even a lifetime without transmitting. Still, you should take precautions: no sex during outbreaks, take anti-HSV drug (e.g. valacyclovir), condoms.