Two very simple words yet they have the ability to impact people in a great way.
"If only" leads to regret. We've all made bad decisions and wish that we could go back and change it but we also know that that isn't possible. So instead of moving on, we dwell on the past. We play that scenario over and over again thinking about what went wrong and what should have happened.
"If only" leads to wondering. A decision that was made in the past can lead to a person wondering what would have changed in the future had they done things differently. When this happens, we tend to over think things and it drives us crazy.
"If only" leads to longing. When something happens that was unwanted or lead to unwanted changes, a person can't help but think about what was or what could have been. Sometimes, even though the outcome was good, a person still might think about what could have happened otherwise.
There are many ways that these two words affects a person but they are all connected in one way or another. Regretting something leads to wondering what could have happened which could end up making a person want something that they used to have or could have had. Regardless of how it affects a person, when the words "if only" are uttered, one can only assume that it's not good, happy, or pleasant thoughts that will follow.