Meet/know/see

When you encounter someone for the first time, you meet him or her. Usually, we say "Pleased to meet you" or "Happy to meet you."

After you've met someone, you know each other. You may not know him or her very well, but you know who the person is. On the other hand, you might know him or her very well. For example, "I've known my best friend for thirteen years."

When you see someone you already know, you can say "I'm happy to see you!" We don't say "happy to meet you" in this case, because we already know the person.