Rowling has made it very clear that attitude and emotion can have effects on
spells, including their power. For example, in POA when Harry performs
'EXPELIARAMUS' on Snape, it isn't just a disarming, it is nearly a
dis-arming. For whatever reason, hatred of DD or hatred of the act, Snape
put a great deal of emotion behind the spell.
The way I see it, magical children in HP universe are some sort of equivalent of sensitive children in real world. It's the people who experience emotions with greater intensity than most, thus affecting reality around them (eg. Breaking of the glass in Sorcerer's Stone in the Zoo, Blowing up of Aunt Marge etc.) I am sure Dudley experienced anger and fear, but his emotions were just not strong enough to affect anything. In most magical practices that I have heard of, real or fictional, emotions are key in spell casting. Depending on the spell you want to produce, you have to focus on certain emotions, otherwise it's all just saying plain words and swinging your wand. I think Rowling too took some inspiration from these practices.
In Harry Potter though, I think the spells that require you control your emotions in such way are learned in latter years at Hogwarts, if ever. (Patronus - you are being attacked by a bunch of Dementors that are sucking all the happiness out of you and you have to remember happy memories... doesn't sound like an easy thing to do to me. Sounds like something that requires immense discipline and years of experience. I think being able to do such spells in a blink of an eye is what makes you what they all call a 'good' witch or wizard.)
And remember, Voldemort did not have happy memories. In fact, I don't think he had the capacity to feel positive emotions. In muggle world he would have suffered from something called psychopathology-inability to experience compassion, love, remorse etc. This is what I think they all refer to when saying that he is weak because he never experienced love. His specter of emotions was limited thus the magic he was able to do was limited and THIS IS WHAT MADE HIM WEAK. The fact that he didn't have access to some powerful magic. This is why he couldn't understand how Harry survived the first time around.
So YES, there are some spells, the most powerful ones, dark or 'white', that depend largely on your emotions, and if emotions are not honest or strong enough the spell may be ineffective.