I definitely don't believe in the idea of there being one soul mate for each of us, nor in the concept of a 'one true love'.
For me, love is not a single noun, describing a single state of being. Love changes along with us. Our ability to feel, accept and give love changes as we do.
I believe that when I love someone, I forever give them a part of me. However, I can't run out of parts to give, as the space is always filled again. This means that if we use the analogy of a heart, when I give a piece of my heart away, the piece will eventually grow back, but it will be slightly different. It may be scarred, larger or smaller, but always replaced.
This means that everytime I love someone new, the love comes from a unique place. The love will be different to past loves, as I am now a different person. Not necessarily less, or more; just different.