Why is fellowship so important?

Genesis 2:18: Then the Lord God said, “It is not good that the man should be alone; I will make him a helper fit for him.”

From the very beginning God said it isn't good for man to be alone. While He made a wife for Adam, which is very important, He also created someone for Adam to have fellowship with. 

Hebrews 10:25: not neglecting to meet together, as is the habit of some, but encouraging one another, and all the more as you see the Day drawing near.

We should be meeting together. We learn best in fellowship than we do by ourselves.

Ecclesiastes 4:9-12: Two are better than one, because they have a good reward for their toil. For if they fall, one will lift up his fellow. But woe to him who is alone when he falls and has not another to lift him up! Again, if two lie together, they keep warm, but how can one keep warm alone? And though a man might prevail against one who is alone, two will withstand him—a threefold cord is not quickly broken.

Two is a lot better than one, and three is better than two.  Having a friend with you, you can lean on your friend for help. It is a lot easier to deal with something hard with a friend by your side.

Proverbs 27:17: Iron sharpens iron, and one man sharpens another.

 We need one another to build us up, and to make us stronger.

Galations 6:2: Bear one another's burdens, and so fulfill the law of Christ.

 We are supposed to bear with one anothers burdens, because we are supposed to love people, and when we love someone we want to help with their burdens.

John 15:12: “This is my commandment, that you love one another as I have loved you.

If you love people as you are commanded to, you will want  to spend time with them, so, fellowship is important to build each other up.