To perform effective magic three necessities must be present: the need, the emotion, and the knowledge.
The need is simple. You wake up one morning with a splitting headache you can’t get rid of. Or you may find out you need a hundred dollars by the end of the month. A friend may be looking for a new love. In all cases, a need exists.
Need should not be confused with desire. Desires are often passing; what one desires one morning may be pushed aside the next morning for something else. A desire is a whim; a need is a deeply-felt, important, all-consuming state.
The emotion, too, is clear. You may need a job, for instance, but if you are not emotionally involved in the seeking of employment, worried or anxious or fretting, all the spells in the world won’t bring it to you.
This is why it is sometimes fruitless to do spells for others, unless you can feel the same need that they feel—emotionally.
The knowledge constitutes the body of magical lore. In other words, a spell or ritual, or the basic theories behind them which allow you to make your own.