Trying to Launch

What I expected to happen was it would work and I would be so happy!:

*What actually happened was my friends said they have Minecraft 1.8.7 and I was quite jealous so I tried out the demo before I could buy it but *
Describe what happened here when it was launcing I was so excited!! Unfortunately, it caused an error when launching and my heart was broken!

Please Mojang, I know that you are the best at making games but I'd like you to help me fix the bug and explain to me about the problem,

Thanks.