When Jesus said this, He was encouraging His followers to pray, knowing that God would answer them well. But one could look at this from another angle. What about after we've prayed, and we don't receive the answer we were expecting? What if, when we pray, we receive an answer we don't particularly like?
I remember when I was a child, around 8 years old. I wanted to be strong, and I enjoyed watching a cartoon called Popeye. Popeye was a sailor, and his character centers around the fact that whenever he eats a can of spinach, he immediately turns into a very powerful individual. So, naturally, I asked my mother to buy me some spinach.
Turns out, real spinach is nothing like the cartoon. It didn't come in a can, for one thing. Nor could it be eaten in one bite. And it tasted awful. But the worst part was, my muscles didn't triple in size in 3 seconds flat. But there's a reason parents encourage you to "eat your greens". Spinach is a healthy food, and so including it in your diet is not a bad thing. Certainly not as bad as my 8 year old mind told me.
Just as my mother answered my request, the Lord answers ours. Just as I didn't like the result, we often don't like the results of our prayers. But Jesus says to us that just as we don't give scorpions to our children when they ask for eggs, so also does God not give bad gifts to His Church. If you ask the Lord for a good thing, He will answer with a good thing. If that good thing is not what you expected, trust Him anyway. He knows what He's doing.