Is the learner himself/herself a factor to be considered? And why?

I am not
sure whether you mean this question from a teachers perspective or a students perspective.  I
think that either way, a learner needs to be an active participant in the learning process. 
Learning is not a passive affair.  Sometimes students think that all they have to do is sit back
and listen to the teacher.  That is not the way at all.  Learning is about interacting with the
material, and that is something that the student has to play an active role in. 

The first thing a student has to do is listen. It may seem silly, but if you arent
paying attention in class, you are doomed.  If you are reading, you have to listen in your head,
I suppose.  Basically, you have to process the information.  An active learner will take notes,
and then ask questions.  You can ask questions in a notebook or out loud.  If you write your
question down, you can ask it later. 

If you are talking about teachers
designing lessons, learners are definitely a factor to be considered.  Obviously, a teacher
cannot design a lesson for each learner.  What a teacher can do is design lessons with groups in
mind and with the audience in mind.  Teachers can include pop culture references in the lesson,
for example.  Teachers can also use local locations and history, people, or trends in their
lessons.  This connects the classroom to the words students know, and makes things more
interesting.

Differentiating lessons is also a useful tool for teachers.
 That means that teachers can vary lessons based on students in the classroom by grouping
students.  Some students learn faster than others, and they get a different lesson with
enrichment, for example.  Other students who need more help might get review.  Differentiation
allows teachers to reach everyone's needs more easily.