When it comes down to it; you’ll never know people as well as you think. Human nature at its core is self serving so keep that in mind when you allow people into your life. Remember that you can never truly know someone; it would require a supernatural ability; to read minds and correctly predict reactions.
Love, security, understanding and compassion can all be faked and once your dealing with these things outside of yourself; it can all be taken away on the whimsy of someone elses flawed emotions.
Building successful relationships romantic and other wise calls upon you as a person to trust and thing outside of yourself, something that you hope the other person is doing as well. When they do not, and the relationship because parasitic, its time for you to move on. Relationships should be mutually benefitting with at least a slight foundation of trust and they should be based on equal imput. When the scales are tipped in either direction; you putting in to little or too much is when it fails and people get hurt.
Also remember that people only do what you allow them.