In action movies, the hero doesn't mind destroying the aircraft, road or bridge he just crossed, because it's always a one-way journey.
Retreating armies used to burn bridges as they crossed them so those in pursuit couldn't follow.
And that very mindset, the mindset of, "I am so intent on my goal that I am willing to push through this person, push through this relationship, push through this interaction, whatever it takes," is precisely how we burn our bridges.
The difference, of course, is that life is long and very few paths are only one way. You will need to come around here again.
A bridge well-crossed gets better over time. When you need to break it down to push through, you've not only hurt the person you trampled on, you've hurt your reputation.