As there is no evidence to the contrary, we can rule out the existence of a supernatural force or being that would give our lives a meaning. This means that the meaning of our lives is defined by what we ourselves make it out te be, what we want to achieve, be it winning a million dollars, becoming president of the world, cure cancer or to simply be with your loved ones.
If you follow it to the end, the meaning of life is to get the most enjoyment out of it, however you do that. Depending on genome and personal experience, everybody has different preferences, different means to achieve enjoyment from.
If fantasizing about a girl that doesn't exist enriches your life without having any side effects that might result in the opposite along the way, there is little reason not to do it. What you must do now is question if this is the case. I can't read your mind.
...yet.
Luckily, we are more or less hard-wired to, eventually, take care for one or more people and to get enjoyment out of the enjoyment of these people. How strong this effect is and to how much people iot applies differs wildly from human to human through the parameter explained in the previous paragraph, though. Now, the optimal thing to do would be to condition everybody from birth onwards to enjoy exactly what they are supposed to do in their later lives. That way they enjoy themselves while maximizing the enjoyment of others, creating a glorious utopia where everybody is happy and lives together in harmony harmony oh love.
A brave new world!
...I really need to read that book some time.
