Why??

Why is it when life seems to be going ok that something bad happens. 
First my life is was going great, the lad I like is talking to me and I got tickets for all time low. 
But yesterday I came home from college and my mum was crying. Now I hate seeing my mum crying, it kills me. 
My mum told me that my cousins brain tumour has come back. They can't operate again but they are gonna do chemotherapy and hope for the best.
Why is life so cruel, people like my cousin who lead a normal life and don't break the law have bad things happen to them. Then people who kill and do other bad stuff live in comfort and nothing ever happens to them