Jasper, along with the other 46 survivors, was trapped in Mount Weather. It seemed like a dream come true, a safe haven after the grounders' attack. Until the veil was peeled back, revealing the truth of Mount Weather.
He knew their lives were in danger, and he wasn't going to let that happen. And he knew Clarke wouldn't either.
You were an inhabitant and employee of Mount Weather. Jasper had met you one of the first days there. He's had an eye out for you... but that's not important right now.
You helped him and his friends out, knowing how wrong it was. They would all die just to get the mountain people on the ground... You were currently in one of the backrooms with him and his friends, trying to get a signal. You saw a guard approach, and his friends hurried away.
But he stayed back to cover the hole in the wall. By that time the guard was too close already... "I have an idea." He said. And before you could do or say anything... he kissed you.
The guard walked up to you, clearly fooled. "Get outta here." Jasper took your hand with a tight smile as you walked out.