Hello, I am available to assist you. Welcome!
I am a mental health therapist, and I happen to also be 50- I know exactly what you mean about the "quiet life".
Like you said, living a life of socializing and wanting to please others- that can be exhausting. We come to a point in our life- let's say half way at about 50- that's if we live to the ripe ole age of 100, that we are more introspective, more time spent on reflection.
I'm hearing you say you are happy, enjoy time spent alone. Do you spend most of your time at home, do you work outside of the home?
I'm not hearing you say you are lonely and depressed. I think a psychiatrist certainly would question you more about this, and or ask if you are "repressing" or avoiding something.
It sounds like most of your struggle relates to what society or a doctor would say about this isolation. You may more a more introverted person, getting more from your time alone, vs. and extrovert that gets more satisfaction out of being with others socially.
Concern would come if you were feeling more lonely, sad, depressed, anxious, etc. I'm not hearing you report any of these symptoms. If anything it sounds like you are at a place in your life where you feel more at peace. That sounds like a peaceful and content place to be.
We've spent 50 years of our life in the busy, active, and overstimulated world, I'd say you have earned the right to spend that time alone, especially when it brings you such pleasure.
It also sounds like you certainly have the option to go out socially if you so desire. For now enjoy life on your terms. Who knows what is to come in the future- maybe that friend will come along who you feel more excited about, more of a desire to be with someone, to be more socially active. In therapy and in psychiatry we treat symptoms- and I don't think you have any to worry about. You asked what happened with such a change, it's likely that you are more comfortable in your own skin, and proving something to someone, or gaining approval from someone is not as important as it once was. At some point in our life we realize it really does not matter what others think, it comes down to "can I live with myself". I'd say you are in that place of more self acceptance and peace.
Something to consider if you do wonder if there is something more underlying this- is to ask yourself, is it a way to avoid, and or protect yourself from expected hurt? Does the thought of being social bring a sense of anxiety and fear? You did not post anything indicating a yes to those questions- so carry on in your life- happy as you are.
I don't think forcing something that doesn't feel natural is productive, so enjoy your quiet and content life. If you do start to feel lonely, sad, etc. that is your cue to venture out .It's when we struggle in life when we need others for support. Maintaining some contact with others, friends or family, will provide you with the option of seeking out others if and when you need them.
I notice you are off line. If you have additional comments or questions please post them. I will be notified if you respond and will get back to you asap. I'm on line for just a bit longer. It is 11:00 p.m. my time- greetings from the USA. Thank you for your post and look forward to corresponding with you at another time.