Remus was terrified of the bike. Even though Sirius had a good point-- a flying motorcycle was less likely to be hit by other vehicles. He didn't like motorcycles and he didn't like flying, so combine the two and you just have a nightmare for Remus.
But he gave into a ride with Sirius. Sirius had his reassurances and well, he was Sirius. Remus couldn't say no to him.
When there was a knock at his window, Remus didn't even think to look up from his book. But then it occurred to him he was on the twenty-fourth floor. He dropped his book and yanked his window open.
"Sirius!" He stopped himself from shouting, stopped himself from spitting out every little worry he had.