Hi Kays - I'm not sure I understand the question completely. Do you
mean "why the reflexive pronoun remains «me» when it's in the
infinitive", or "why the infinitive is used at all"?
If it's the first question:
There is a rule in French that verbs of "preference" don't take on
a preposition. So when forming a compound verb structure with one
of these verbs (like aimer, détester,
préférer, etc.), your structure is just: verb +
Ex: J'aime manger des mangues. (I love to eat mangoes).
When using a pronominal verb as the infinitive, the reflexive
pronoun still has to agree with the subject (je-me, tu-te,
il/elle/on-se, nous-nous, vous-vous, ils/elles-se).
Ex: Mes parents aiment s'embrasser en public,
c'est gênant ! (My parents love to kiss in public, it's
J'aime me promener. (I love to walk).
Mes parents préfèrent se promener dans la forêt. (My
parents prefer to walk in the forest.)
If it's the second question, they mean different things.
Je me promène. => I walk.
J'aime me promener. => I love to walk.