If you use a private key, it lives on your hard drive or in another secret place. To communicate with encrypted messages, two people need to agree on what key they're going to use to code and decode a message. If this communication gets neglected, the message recipient won't be able to make heads or tails of the encrypted information. It's like asking someone to feed your cat while you're away, but forgetting to give them the house key. There are two types of encryption, symmetric and asymmetric, and they use different types of keys.
Symmetric encryption uses a private key for both encoding and decoding a message.
Asymmetric encryption uses a private key for the encoding and a public key for the decoding, or vice versa. Let's say Joe sends a message to Mary encrypted with his private key. Because Joe's private key is related to his public key, he tells Mary what his public key is, and she uses that to decode his message. But Mary can't figure out what Joe's private key is by his public one, so he can use that private key again and again to communicate with other people. Not at all.