לזכות הרב מרדכי גימפל בן חיה אביגיל שה' יוציאנו מאפילה לאורה ומשיעבוד לגאולה
There are two ways to fulfill a mitzva. One way is to do so because it is written down that one must fulfill this mitzva. So a person does it. Even if it is performed with enthusiasm it is still being done because the text says so. This is much better than doing it for ulterior motives but still not [I believe] the ideal.
The ideal is to fulfill a mitzva because I want to do the mitzva. Example: Many of us eat breakfast. In fact the gemara says that we should eat breakfast. So what is our attitude towards our doughnut and coffee? Is it "The gemara says to eat so I am fulfilling the words of Chazal"? Or is it "I really enjoy my morning doughnut and coffee and when I eat it I am fulfilling the words of Chazal". Meaning, I naturally want to do this act anyway but since it is written in the gemara my intention is to eat in holiness in compliance with the words of our Sages.
The latter way is the ideal. The Torah should not be a set of superimposed rules but something that flows from within.
If I love my friend because there is a pasuk that commands us to do so then my love is lacking. I should love my friend naturally and deeply and think "I am fulfilling the mitzva of loving my friend and this is a sweet mitzva".
There is a SHOCKING Rashi [נדה י"ז. ד"ה אונס אונס שינה]. He writes about the act of conjugal relations
"מתוך שהוא נאנס בשינה אינה מתאווה לה כל כך ומשמש לקיום מצות עונה בעלמא או לרצותה וקץ בה והוא מבני תשע מדות דאמרו בנדרים כ,ב".
Since he is exceedingly tired he does not desire her so much and has relations JUST IN ORDER TO FULFILL THE MITZVA OF ONAH OR TO APPEASE HER and is disgusted by her, the children will be spiritually blemished [see Nedarim 20b בני תשע מדות].
This is a revolution and completely counter intuitive. One would think that specifically the mitzva of Onah should be fulfilled only because it is a mitzva with zero personal interest. Isn't that קדושה? Comes Rashi and says "No". If one's intention is JUST for the mitzva or to make her feel good then the children that result from this union will be blemished. There must be a natural element of desire.
Of course, one should have this natural desire because Rashi says so. If Rashi would tell us to obliterate it and only have intention for the mitzva then we would do that as well. But now that Rashi tells us to to desire in a natural way - we must do that.
This can be applied to all mitzvos. One could and should enjoy the intellectual pleasure of learning Torah while appreciating that by deeply thinking and analyzing with great interest - he is immersing himself in the word of Hashem, fulfilling His will and sanctifying himself. But if he just learns because the Shulchan Aruch says that one must - he is missing out on a complete fulfillment of the mitzva.
There is MUCH MORE to say - maybe for another time....