Praying for a Sign

Why do people always pray and ask God to show them a sign or a miracle to help them believe in Him or trust Him?

I find myself doing this all the time without even realizing it.
"God just show me that this decision I made was the right one"
"God just give me a sign that you are there listening to my prayers"
"God show me a sign that you care and love me"
"Please God, just one little sign just so I can know and believe"

This past semester I prayed to God asking Him for a sign that everything would be okay. He gave me a sign back, actually gave me 21 signs back in only two weeks. He placed the verse Jeremiah 29:11 into my life 21 times in some of the craziest ways. Here are just some:
1. I googled verses about trusting in God and picked it as my favorite
2. I wrote it down in my journal
3. My friend Amanda's cousin adopted a baby named Jeremiah
4. I opened the book God Girl and the first sentence I read was Jeremiah 29:11
5. A man named Dr. Finch said to a small group of people and I that he loved the verse Jeremiah 29:11
6. I went on a Women's Retreat with CRU and the speaker Kerri quoted it
7. My roommate was talking on the phone and quoted it to her boyfriend
8. I called my friend Evan to tell her this story and she texts me a couple hours later saying she just met a guy named Jeremiah
9. My friend Chandler texts me to tell me she keeps seeing Jeremiah 29:11 everywhere!
10. I called my mom to tell her the story and that day a guy named Jeremiah came to measure our kitchen for new counter top

So while this was happening to me I said to myself I need to trust God. It is time to listen to Him. It took a sign like that for me to believe that God was listening to my prayers and to show that He did care about me. But shouldn't I have trusted Him and known he listened already? Why did it take a sign for me to believe?

Jesus said that humans need signs to believe. 

John 4: 48 “Unless you people see signs and wonders,” Jesus told him, “you will never believe.”

It is not just the humans of our generation who demand signs, I Corinthians 1:22 says, "Jews demand signs and Greeks look for wisdom", so people have always wanted a sign to believe. 

But why do we think this way? Why do we need proof to believe? Isn't the bible enough proof? Why can't we just have faith? 

I wish I knew the answers to these questions. All I know is that God listened to my prayers and showed me that he cares and that I need to trust Him and His plans for my life. So that is what I will do.