There is something about getting around new Christians that can excite and revive our faith. It's like getting around consumers passionate about a product. No one pays me to promote my iPhone. I do it because I love it and I can't keep it in. This same "consumer excitement" occurs every time someone new chooses to follow Jesus and receives his forgiveness. It's incredible. They remind us "mature" Christians of the joys of following Christ and the difference he makes from living a worldly life. Sometimes we get so far separated from they way we used to live before Christ (or we grew up in the Church and didn't have to experience it to a certain extant), that the only way to keep it fresh is to see this new excitement and fresh faith displayed in a new believer.
This is one of the reasons I cherish my call from God to be an evangelist. It is such a help to my faith to see people's lives being changed by Christ on a regular basis. I've thought frequently that in the bad times of our lives we tend to remember all our bad memories, and in the good times of life all we can often recall is the good nostalgia. It's so hard to keep a constant remembrance of all the miracles and workings God has done in our lives. Bottom line: I think we (humanity) have bad memories. Psalms speaks about this. Seeing someone set free from a lifetime of sin and bondage, or the intellectual who finds new hope and joy in trusting Christ can be the exact reminder we needed to recall all the benefits of serving God. In a way, it keeps it fresh.