You're in a queue to buy a train ticket. If you don't buy your ticket in the next 3 minutes you will miss your train. You need to find a way to be served next.
Which of these would most likely get you a place at the front of the queue?
"Please could I jump the queue and go before you?"
"I'm about to miss my train. Please could I jump the queue and go before you?"
"If I don't get this train I'm going to be late for an interview that could change my life. Please could I jump the queue and go before you?"
Naturally, 3 will work better than 2. 2 will work better than 1.
It's simple. If you need something from someone, you can massively improve your chances of success if you explain why.
It's a wonder why so many people forget this when they get to work. I think it's because in the examples above we are relying purely on a social exchange to get what we need. At work, we are in a transactional environment, people of being paid to do what other people ask, heirachy and power plays interfere - this means we sometimes get lazy with our asks.
Give me a reason and I'll be more likely to do it.