In the beginning of Be'haaloscha, Rashi explains that Aharon lit the menorah in the same fashion each time. Which meforash elaborates on this pshat, explaining that he did it with the same vigor throughout his life and never viewed it as a monotonous task?
The שפת אמת (תרל"ה) explains this thought.
He writes that when it comes to a repetitive Mitzva, it is human nature to be passionate about doing it at first, but as time goes on, this passion is forgotten. Not so Aharon Hakohen, his passion stayed the same every day.
Additionally, being that the first passion is the best, he was able to acquire higher levels of spirituality with each kindling.