I believe the power of a witch or wizards magic has to do with their emotional state and well being.
Let me try and explain-
Lily sacrifices herself in order to protect Harry from Voldemort, a protection that Dumbledore explicitly says has to do with her love(Yes, not loving someone that much would not make you sacrifice yourself, but then you wouldn't be able to use that "old magic" anyway).
In GoF, Harry needs to draw on his happiest memory to be able to cast expecto patronum, and the happier the memory/thought, the more powerful the patronus.
Another example is when Barty Crouch Jr. explains to the students in the DADA class that even though they all pointed their wands at him and uttered the words for avada kedavra, he wouldn't get so much as a nosebleed- which I think is because none of them would truly want him dead(wanting someone to die is usually because of hate/jealousy/anger etc):
Avada Kedavra’s a curse that needs a powerful bit of magic behind it — you could all get your wands out now and point them at me and say the words, and I doubt I’d get so much as a nosebleed. - Barty Crouch Jr. -Harry Potter GoF
One more example is when Harry chases down Bellatrix in the ministry of magic after she killed Sirius Black.(Order of the Pheonix) He tries the cruciatus curse on her, which ultimately fails because he didnt have the correct "emotion" paired with that spell(he has to mean it. In other words, he must want to hurt the target, even get some form of pleasure from inflicting pain).
Hatred rose in Harry such as he had never known before;he flung himself out from behind the fountain and bellowed,'Crucio!'
Bellatrix screamed:the spell had knocked her off her feet,but she did not writhe and shriek with pain as Neville had- she was already back on her feet,breathless,no longer laughing.Harry dodged behind the golden fountain again.Her counter-spell hit the head of the handsome wizard,which was blown off and landed twenty feet away,gouging some long scratches into the wooden floor.
'Never used an Unforgivable Curse before have you boy?'she yelled.She had abandoned her baby voice now. You need to mean them,Potter!You need to really want to cause pain-to enjoy it-righteous anger won't hurt me for long-I'll show you how it is done,shall I?I'll give you a lesson-'
Chapter 36 The Only One He Ever Feared Harry Potter and the Order Of The Phoenix.
I am sure there are a few more examples in the books that could be used, but these are a few that I remember of the top of my head.
Dumbledore, Voldemort and Harry (Three of the main characters) all have something in common- a messed up childhood. It is my believe that because of this, their (innate) power are amplified a bit by the powerful emotions that they experience thanks to their history.
Harry had to use a memory of his parents before their tragic fate in order to create a corporeal patronus- would he have been able to do so if they were still alive and well?
Sorry for not having the relevant quotes for my claims- I will add them once I get home tonight as I do not have access to the books from work. If anyone feels they want to add the quotes- be my guest! :D