Because the things we do for no reason, are the most meaningful things we ever do. Life often gives us little moments like these. It is better to take them on its merit without questioning the rationale and enjoy while it lasts, after all life is short. They're what gives life meaning and they're what makes life worth living. They're what makes us human, even.
It's simple. They didn't feel like taking the effort to explain it to you and/or they never really thought about why they did it. Sometimes the things we do for someone are unconditional, we love to do those things because it gives us happiness to see them happy, At that moment it does not matter what we feel or what we want, they become a priority before yourself. In live we experience such love many times, but little do we take time to appreciate those, we take them for granted. Do you know the reason behind why your Mother loves you from the very first day when you were born. Was there any reason behind that? No!... Do you know the reason behind why your that Best friend does so many things for you without you even knowing or appreciating them? No! ... Do you know the reason behind why someone who truly loves you gets sad if you're sad and find their happiness in your happiness? No!
All this does not have a reason, it's all this is because of love. Love can be expressed in many ways and to anyone. It doesn’t require any reason because the way u spread love could make someone else’s day. Love someone without any expectation with heart, eventually it will definitely come back to you.