"Love" in this quote can be said as "passion" too. If you love your job or work, then you do it with all of your heart. But if you don't, you just do it as "duty", likes only cares about what the teaching topic, explain it to students, give exam, evaluate it, and etc. But a teacher who teach as "love" or "passion", they cares their students, help them to learn, encourage and appreciate the student achievement, help the student with difficulties to learn, motivates the student to loves their learning. Student known well the type of their teachers. Just ask the children around you who is the teacher they loves and hate. you will find that the teacher who loves most surely is the teacher who teach with love then duty. And, the teacher they hate is who teach just as "duty".
If you are parent, when you teach or help your child to learn such homework, which one is your types?