It can be tricky business to find ways to implement the motto "sharing is caring" while also setting firm and healthy boundaries for yourself. While sharing your resources, both emotional and physical, with those close to you is a net positive, forming the ability to say no to people willing to exploit you is essential.
As with most emotional skills, it's far easier to set healthy boundaries in adulthood if you're given the tools as a child. Sadly, this is one of the many areas where American culture fails children. There is a pretty large disconnect between the ways we urge children to be open with everyone, through hugs and emotional performance, and the skills they'll need as adults.