From where do we learn that it is occasionally difficult to fulfill Mitzvot? From the Mitzvah of Brit Milah, in which we circumsize an eight-day-old newborn and he cries.
If Hashem wanted us to have a Brit Milah, why didn't he create an easier way without pain? For example, we could have been born already circumsized, or the cutting could be painless like cutting fingernails or getting a haircut.
The reason is to teach us from the youngest age that fulfilling Mitzvot is occasionally difficult, but it is worth it!
But it is important not to overemphasize the difficult aspect. After a Brit Milah, the baby calms down after a short time. A French author wrote many years ago that a Christian baby cries for a lot longer after being sprinkled with cold water during a Baptism. Furthermore, a baby suffers a lot more when he gets soap in his eyes in the bath, and yet we don't stop giving him baths.
We must therefore understand that fulfilling Mitzvot is occasionally difficult. We must gird ourselves with strength and courage since we will come to understand that it is worthwhile!