Buddhism teaches people that the real source of happiness is inner peace. If our mind is peaceful, we shall be happy all the time, regardless of external conditions, but if it is disturbed or troubled in any way, we shall never be happy, no matter how good our external conditions may be. External conditions can only make us happy if our mind is peaceful. We can understand this through our own experience. For instance, even if we are in the most beautiful surroundings and have everything we need, the moment we get angry any happiness we may have disappears. This is because anger has destroyed our inner peace.
"Mind precedes all things; mind is their chief, mind is their maker. If one speaks or does a deed with a mind that is pure within, happiness then follows along like a never departing shadow." ~ Dhammapada
We can see from this that if we want true, lasting happiness we need to develop and maintain a special experience of inner peace. The only way we can do this is by training our mind through spiritual practice – gradually reducing and eliminating our negative, disturbed states of mind and replacing them with positive, peaceful states.