Remembering a time when you felt happy or loving, peaceful; such as holding a small child or pet, can help bring up a warm glowing feeling in the center of your chest. Smiling, both externally and internally can also bring up this feeling of love. Some meditations practice using a mantra such as may I be peaceful, may I feel love, or the feeling of the breath in the body, but the key element that few meditations share is that the pleasant feeling is the most important part of the beginning of the meditation. The thought, mantra or external meditation object will not bring you joy or change your brain chemistry. A pleasant feeling is imperative.
The second problem with other, even deeper meditations is they teach maintaining an absolute focus on that object of meditation, by shutting out all other feelings and thoughts by sheer will. Mindfulness meditation is simply about noticing another feeling, sensation or thought that arise in your body and mind, and then relaxing both the body and the tension in your mind, and returning to the pleasant feeling. Nature abhors a vacuum. Once you let go of the thought or other feeling by relaxing the mind and body, you need to bring up that pleasant feeling to fill the void. But true healing meditation is about mental awareness; meditation is not only creating a constant pleasant feeling, but building the ability to perceive when the mind moves away from what you intend to focus on (a pleasant feeling), and to relax and come back to the feeling more and more often.
This gives the meditator control over his or her mind, re-creating pleasant feelings and giving the meditator the ability at will to use his or her mind, instead of being occupied by more detrimental thoughts and feelings.