When you are happy you are at an all time high and don't want the journey to ever end. But everything good must come to an end and you must accept the fact that it is unrealistic to feel that way forever. I'm not saying you'll never feel that way again, I wish everyone a happy life, but you are meant to feel so much more in life.
Also would you appreciate the happiness as much if you never experienced a feeling of sadness or depression? My genuine and honest answer is no. You would also be stuck in a fantasy, lose your passion, lose substance, and not grow in individuality. Next time you feel new and unfamiliar feelings, appreciate them instead of fear them. Let them in, take them in, explore them, embrace them, and put them towards your passion. Learn to find peace in every situation you are placed in. Most importantly, remind yourself that you are feeling that way for a reason. A reason that may be big or small, but it's still a reason.
Through experiencing new feelings I've learned from my own mind that once I jump out of a depressed couple of days I suddenly have a newly found appreciation and love for many things in life. I discover why I was feeing that way and suddenly become even happier with my own mind and body. Even though it's not fun to be sad or depressed, I know I was meant to feel that way at that time for some particular reason. I Aim to discover what feelings truly are. They are beautiful. They are raw. And they are most importantly a 100% real picture of person's true character.