1. I know that I should look 'at' the good side of my life.
2. I know that I should look 'on' the good side of my life.
[Which preposition should we use? Are both okay?]
3. Try to look on the bright side.
4. Try to look at the bright side.
[#3 is a common expression. Can we substitute 'at' for 'on'? Are both interchangeable?]