Why am I no longer interested in things that once made me so happy?

This is a symptom of depression. If anything really upsetting has happened to you recently, or you're feeling really stressed, those may be some causes.

That can be to do with depression as it drains the enjoyment out of things, but it could also just be because you're growing out of stuff

There's a multitude of reasons as to why you no longer gain any sort of happiness from things past such as a negative memory attached to thing you once loved, as you grew older you grew out of that "phase" or you could just be going through a rough patch that makes you feel as though you're unhappy with these things which could symbolise that you need to get some help from someone. If you're willing to speak in more depth about it, I can highly recommend speaking with a Listener one-on-one so they can understand the situation and help you to come to your own conclusion of the situation.

We all change so do the things we enjoy doing and our ideas of happiness. Try to see development here.

Maybe it is time to try something new. Do new things and travel to new places. That way, there will be many opportunities to learn new things and find more things that make you happy.

I think over time, it was mostly me growing out of certain toys and activities. More recently, I've noticed that I stopped caring about a few other things that seemed more important, but I know that over time I shall find their importance once more. After all, I kinda need to if I want to do something with my life.

Why not? Life never promised us an eternal picnic in the park. When we were born many (if not most) of us were dealt a pretty crappy hand! Very few people in life win right away and out of the gate. It takes patience, time, gentleness and love to keep playing, to get ahead in the game of life. Sometimes we are behind and sometimes we are ahead but when everything is an opportunity for growth then we can become grateful and enjoy the ride. Yes; anger, fear, sadness may also be there but if one is willing to do the work and has had enough then no more time is wasted on sitting around complaining and procrastinating. :)

There are many reasons for this to happen, it can be a symptom to depression or simply growing older, sometimes our tastes change and we develop new interests at the extant of older ones. I think it's important to make the difference between a lack of motivation in general or a lost of interests in only certain things.

Life can change so dramatically that something you once loved doesn't mean as much now. Maybe try to love something else

Sometimes the journey is the most important thing in life. When you got to where you wanted to be to make you happy, what else is there? the old adage about careful what you wish for is very true in my experience.