Pirkei Avot, 6:4
The above is explained by the Ben Ish Chai (Taken from משל ונמשל) with the following parable:
Two donkeys were carrying loads. One donkey was struggling with an extremely heavy load of salt, while the other was carrying a large sponge, that was so light that he didn't feel any discomfort. The donkey carrying the sponge walked around merrily, running up the mountains and jumping over the hills. The donkey carrying the salt struggled as the salt was so heavy.
Due to the trail they had chosen, the donkeys had to cross a river. When the donkey carrying the salt entered the river, the salt started to melt. As the salt melted away, the donkey started to cross the river happily and peacefully with the emptying sack. When the donkey carrying the sponge entered the water, the sponge got heavier and heavier. So heavy was the sponge that the donkey's legs could no longer carry him, and he drowned.
The journey that the donkey carrying the salt took was for his benefit, because it gave him peace and serenity after the heaviness had melted away. The donkey carrying the sponge had an easy time at the beginning, not even feeling the weight, but when he entered the water - it led to too much toil. The donkey couldn't stand the labor, and that was the reason he died.
Instead of writing up the Ben Ish Chai's analysis about the parable, I think I'd like to sum it up as follows: One shouldn't be intimidated, or turned off, by the weight or difficulty of Torah study - it is for our benefit, both now and in the future.