Thank God for my ability to choose

You have a right and freedom to expect certain thing to be certain way.

However, sometimes the thing becomes certain way which you don't want it to be for any reason, then you may be disappointed as an initial reaction.

Now you have some options:
1. be angry, be disappointed, be frustrated, blaming someone/something, etc. [unhappy]
2. be grateful, try to learn something out of it, and grow, and appreciate your growth [happy]
3. even from an initial reaction, you do the option 2 [happy]

The bottom line is:
You may not have a control on whether things happen according to your expectation or not, but
To become happy or not--nobody can control but YOU!

That's the amazing value of "your choice"