There is no good word for the opposite of lonesome.
One might be tempted to suggest togetherness or contentment , but the fact that these two other words bear definitions unrelated to each other perfectly displays why lonesome cannot be properly mirrored. It does not mean solitude, nor alone, nor lonely, although lonesome can contain all of those words in itself.
Lonesome means a state of being apart. Of being other. Alone-some.