Unconditional Love

Why is it that when I read about true love, I can imagine what it is and understand what the gurus say it is, and I believe that I am trying to practice it wherever possible – but in the context of relationships I find that when I have a disagreement with my partner all sense of ‘loving her’ disappears. Surely if I felt true love for her as I believe I do, and as I seem to feel constantly for my parents for example, It would not disappear just because of an argument?