I love babies. Toddlers get more difficult and not as sweet as babies in the cuddle sense, but there is something special with toddlers too. Same as kids age, that is why women get 'baby fever'.
I think that brand new passion is like baby fever. We miss that sweet newness, that excitement and discovery. But we have to embrace the changes and the tender moments and progression as the relationship grows and becomes deeper. Focus on enjoying the moment, and don't keep looking back. There is nothing back there but memories. And when you keep doing that, you miss the good stuff in the now and possibly throw away the special stuff in the future. It's all a matter of perspective.
However, even mature relationships can still be passionate and fulfilling. Keep up the dating and the fun. My husband and I have been together 10 years, we have plenty of monotonous moments and routines, but we are still madly in love and we are careful to nurture our relationship. It's when that invested time and care stops, when the whole thing becomes stale.