Think of it like this. Every thought you have has an effect on how you feel. Those effects are often subtle, so one thought may have very little perceptible effect. Thoughts are like drips of water falling on stone. Over time they carve deep channels. (Some thoughts of course can have a very big emotional effect, but they’re relatively rare). But the barely perceptible effects of all these thoughts are cumulative — over time our thoughts strongly affect the way our emotions arise.
Most of the time we’re not even particularly conscious of what we’re thinking, never mind what effect it has on our emotions and attitudes. This is why also doing the other main meditation practice we teach on this site — the Mindfulness of Breathing — is so important. We need mindfulness in order to see how our mind works — how thoughts and emotions arise.
In the Metta Bhavana meditation practice we’re becoming more conscious of how our thoughts affect our emotions. We’re consciously encouraging words, phrase, images, memories, which will have the effect of reinforcing positive emotions and undermining negative emotions.
This has the short term effect of altering your mood (you can lighten up for a while) and the long-term effect of altering your personality so that you become a more emotionally positive person — less prone to anger and despondency, and more prone to love, empathy, confidence and contentment.
Meditation has even been shown to “rewire” the brain. Through practicing meditation you build new pathways in the brain and actually develop more brain tissue in the parts of the brain connected with a sense of wellbeing.
This all takes time, of course. Those drips have to wear away at the stone of your established habits. But it works. That dripping water is inexorable. Water is stronger than stone. All you have to do is to keep the water dripping by practicing daily.