Best friend issue?

Me and my best friend, Sam are Vet Interns. We work very hard everyday. This internship means the world to both of us, so we try to work hard. Sam and I, and other collegues, have a guidence teacher who guides us through surgerys and other important taks. AKA our boss. One day he comes up to me and days great job with the surgery. You should accompany me for lunch to talk more. Then I said I am not sure that going on a date with my boss would be appropieate. Then he made me laugh and then I said ok. The following 2 months it went form coffee, to lunch, to dinner and to but only once. At work we just act normal, like nothing is going on. It was late one night. Me, Jhon and Sam were the last people to leave. Jhon grabbed my arm and we kissed, then he left. Thats when Sam yells and says, I can't believe you! Then I said I know it's wrong going out with your boss but it's love. Then we get into this big argument about how she likes him! I never knew that!