He reacts like that when you walk into a room because when you walk in, his sun, moon and stars all are coming out at once. The center of his universe is there and he wants her NOW. This is very, very typical for this age, as is the desire for you to carry him around and be in physical contact with him every minute you're in his vision. Why would you label him miserable when the fact is he's both thrilled to see you and terrified you'll leave?
The fact he can be with others and not cry the whole time (until he sees you and screams) is very good. He IS able to be places without you, for a time. Don't expect more than he is developmentally able to handle just now; realize that this is a phase and NOT a negative one; it is normal for children to have this powerful desire to be with one person (usually the main caregiver, in this case, mom) at this stage.
Not going to him at night telegraphs to him the message that the person he is depending on, above all others, to be there for him is not there at night. Remember this: At his age, when you leave a room -- or don't come when he expects you to come -- he truly thinks you no longer exist. He doesn't yet quite have the ability to understand that people still exist when they're out of sight; when you leave you drop off the face of the Earth (hence the intense reaction when you reappear). When you don't come at night, he figures you are never coming at all because you are gone for good. Try to think like he does, not like an adult does.
Go to him. Some parents are going to post that you should let him cry it out. I don't go for that. He's not old enough for a while yet to understand the "lesson" of "You have to comfort yourself, I'm right here but next door." You already know from experience that if you don't go to him, it gets worse. Why let him work himself into a frenzy? Go to him, resettle him, and over time gradually let the time you spend going back to him be less and less. Don't talk to him or engage him during the night, just let him see you are there. He will outgrow this but I believe that not reassuring him at this stage makes a child more insecure, not more secure.
During the day, yes, you can reward the non-screaming times but don't punish the loud reactions. Again, he is in a phase where the universe revolves around you. Give him lots of opportunities for exploration; engage and distract him with good activities and interactive ones where you play with him a lot (kids learn by being talked with, played with, interacted with), and if he's not ready for day care do not send him.
Please don't put a label on him, even just in your own mind, as a "miserable person." He's not making trouble for you with the screaming; he's not being "good" for everyone else and miserable for you on purpose to hurt or challenge you. He is reacting strongly to you because he wants you so badly when you are there, and when you are not there, he settles until he sees you and then he is desperate to have you right there attached to him. If you look on it as his being miserable or naughty, he will pick up on the feeling that you are unhappy with him.
Spend as much time as you can actively interacting with him. Carry him while you can and while he still wants it. If you need to fold clothes, give him a piece of clothing to play with right next to you and talk to him while you do it. Same for any other chore--involve him and do it next to him. He will slow you down, yes, but it wont' be too long before you'll wish he would want to be with you. He will learn and he will also grow more confident because he knows you are there for him. And eventually he will not need you so much. That's more difficult for a parent than you realize just now. But he's not an "unhappy person" -- just someone in a particular stage of his development.