A quote from St. Jerome explaining why we call apon the saints for help, and why they respond to our prayers; "If Apostles and martyrs while still in the body can pray for others, when they ought still to be anxious for themselves, how much more must they do so when once they have won their crowns, overcome, and triumphed? A single man, Moses, oft wins pardon from God for six hundred thousand armed men; and Stephen, the follower of his Lord and the first Christian martyr, entreats pardon for his persecutors (Acts 7:59-60); and when once they have entered on their life with Christ, shall they have less power than before? The Apostle Paul says that two hundred and seventy-six souls were given to him in the ship (Acts 27:37); and when, after his dissolution, he has begun to be with Christ, must he shut his mouth, and be unable to say a word for those who throughout the whole world have believed in his Gospel?"
Fr. Zuhlsdorf's blog, http://wdtprs.com/blog, has an expanded explanation of the why's and how's of the intersession of the saints.