While it is possible for someone else to cast a spell for you, it's more powerful when you cast it yourself. The reason is because when you cast it yourself, it is linked to your energy.
You see, every action (Be it a movement, word, or both) performed in a spell creates energy, and that energy is linked to someone else when the caster selects their target. When you cast it yourself, it is already linked to you. This is why it is more effective to cast them yourself. Because they use your energy, and therefor connect with you better. This is also why when you're casting a curse, you must state your target clearly. If you don't, the curse, being connected to you by your energy, will either disappear or backfire.
I know it's a bit long, but you must know this in order to cast successfully. (So, if you skipped to the bottom, I suggest you read it.)