Why is it awkward between us?

I have a crush on a guy in my apartment building. He is really nice and social. My mom can have long conversations with him and he is very engaged in the conversations he has with her.
I met him today and he said hey how are you and we talked for maybe 30 seconds. Stepped into the elevator and we didn't say a word for almost the entire ride. I couldn't for the life of me think of something to say and I could see that he was thinking just as hard. And I had to break the awkward silence with the weather is great today and we talked about that til the elevator stopped at my floor. Why is it so weird and awkward when we are alone? I really hoped we would find something to talk about and find out more about each other.
Why is it awkward between us?
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