"もし一つだけの笑顔しか持っていないなら、それを愛する人にあげなさい"

Simplified Meaning:

Imagine you have only one smile left for an entire day. It makes the most sense to share that smile with the people who mean the most to you, like your family or close friends. These are the people who support you and care about you deeply, so giving your smile to them shows your appreciation and strengthens your connection with them. For example, think about a time when you were feeling really down. A smile from a loved one, even if it's just a small one, can make a huge difference and lift your spirits. It can remind you that you're loved and not alone. In the same way, your smile can have a big impact on someone you care about, brightening their day and making them feel valued. So, when you have something as precious as a smile, it’s worth giving it to those who matter most, rather than keeping it to yourself or giving it to someone who might not appreciate it as much. This helps to build stronger, more loving relationships.