Why do we not love ourselves? Because we are not raised to love ourselves. The common experience is to be raised feeling numerous failings as a result of the words spoken to us or actions taken by parents, family and friends. And we want desperately to be loved by our parents but feel the love in conditional, at best. To make the point, a recent New Yorker cartoon showed a mother sitting with her arm around her young son, saying, "Heavens no, sweetie – my love for you has tons of conditions.”
The mind absorbs these negative words or actions and perversely makes them and the resulting insecurity our self-image. Most people are in the control of their mind, and so they do not love themselves; many actually loathe themselves.
People may hide these feelings with bravado, humor, and successful accomplishment, but this is just a façade. Deep down these feelings are always there and control our emotions, how we respond to situations.
Not only is not liking yourself a terrible way to go through life, but not loving yourself unconditionally allows all the negative energy within you to flourish. If you loved yourself unconditionally you would not feel insecure or any negative emotion.
So how does one go from not loving oneself or even loathing oneself to loving oneself unconditionally? This is a major shift in your paradigm, but it is possible - challenging, but possible - and the path is clear.
Bottom line, you need to free yourself from the control of your mind and reconnect instead with your true self, which is your heart, your true Buddha nature. This may sound scary since you've identified with your mind your whole life; it's all you know about yourself. But it's actually liberating, freeing,
I have written numerous posts over the years that discuss this process. The three I would recommend reading first are: "How to Free Yourself from the Control of Your Ego-Mind," "Who/What Is Your True Self?" and "The Heart's Embrace." This process is also discussed fully in my book, How to Find Inner Peace.
May you experience peace and happiness.