Why am I so sad when my life is practically perfect?

i think you have low self esteem and don't value yourself enough. that;s why you feel sad. you are being very punitive. very self critical and harsh. beating yourself up for not being grateful etc. its hard to feel alone when you have people around you, but it can and does happen. its usually that you have people around you but you don't really have anyone to TALK to and connect with on a meaningful level. can up open up to your friends and family? can you tell them how you feel? do they listen and empathise?

because we're predisposed to think negatively when it's normal.
try to think of a way to look on the positive side, that is, find the solutions out of the mess.
when you look through the perfect world with a negative lens, it becomes negative/ unperfect.
what you can do is to focus on the happy moments in your life and look at the world with a positive side of view. I know it is difficult for you to lift up the mood, but try your best to lift it up.
and another possible reason could be depression - feeling depressed leads to feeling sad.
a way to get out of sadness is to find positive things to do or watch - can cure depression as well.
everyone can feel alone and worthless at some time but try to chat with friends when you feel you're alone and do things that are worth doing - bringing back the meaning of life. do things that you enjoy doing so as to make it worthwhile.

Focus on YOU, you just mentioned all the 'great things' and they were around you, but you didn't mention you.

So get to know YOU, what are your SKILLS, and TALENTS, do you have PASSIONS, and INTERESTS you can develop, and become better at?

Once you get a better relationship, and familliarity with yourself, you'll feel better. Life is all about balance, and you have great outward factors, but you seem unhappy, or maybe just unfamiliar with your inner-self.