The 30-year-old was named in Warren Gatland's 37-man squad to tour Australia this summer, despite retiring from the England national team a year ago.
"I didn't expect the chance to play international rugby again, to be truthful," Stevens told Sky Sports News.
"I obviously decided not to represent England. I had my reasons and it was the most difficult decision I've ever had to make as a rugby player, but this is a one-off experience that you just can't turn down.
"It's such a huge honour and privilege to play for the British and Irish Lions I just couldn't refuse the chance to do so."
Stevens appeared 39 times for England.