One of Cancer's primary purposes is nurturing. Taking care of yourself and especially others gives you purpose, and makes you feel part of a loving family.
If someone is getting this treatment from you, they can be sure they've passed your test, and been truly accepted into your world.
Sometimes people take advantage of your nurturing ways, and may not return the favor. Establishing healthy boundaries is one way of making sure you don't get too drained in love and relationships. When that happens, you're no good to yourself or anyone else.
So, nurture yourself first, and caring for others comes naturally.