You don’t have to be madly in love with it, or have it be one of your deepest beliefs. It could be an idea that took your fancy yesterday but – for the moment at least - it’s intriguing to you.
You care enough to research it, discover more, try to get a handle on the idea and what lies behind it. Your interest and enthusiasm will come through to your audience. It will give life to your speech. You might not want to take questions about it, but you will have shared your pleasure in the discovery.
Sharing your pleasure is a gift. People will take away the pleasure and the interest as well as whatever facts you have shared. Which do you think they will remember longest?
But sharing an idea that concerns you deeply comes across in a different way. Suppose you believe it extremely important that more people should donate to cancer research. A deeply felt appeal will reach your audience at an emotional level and might well cause people to move towards doing that.
The danger lies in caring so deeply that you become a bit overwhelming on the subject. If you speak about it every chance you get your listeners might get bored with it and tune out. Maybe you unintentionally indicate that this is the only useful avenue for donated money and you risk turning off those who donate to, say, world hunger or animal welfare.
Care enough about your deepest beliefs that you exercise some tact and self restraint. Save your passion for a small number of speeches about it, but make them count. Within a few tightly focused speeches collect all the pent up passion and do a first rate job of making others care as much as you do.
Caring for your topic comes in a range of strengths from temporary enthusiasm to heartfelt belief. Play the whole range as you develop your skills as a speaker. See where your strengths lie and build on them.