If the following sentence is correct:
J'aime le sandwich (I + like + the sandwich)
I expect (I + love + you) to be:
Then, why is it "Je t'aime"?
There are 2 rules that answer your question.
First: subject (direct or indirect one when complement to the verb) ending with 'e' and placed in front of a verb starting with a vowel must be shortened. Thus, 'je', 'me', 'te' and 'se' will become " j' ", " m' ", " t' " and " s' " when in front of verb starting with a vowel. "Je aime le chocolat" is incorrect => "J' aime le chocolat". "Tu me as fais peur" is incorrect => "Tu m' a fais peur".
Second: when you shorten a direct complement of a verb, you must replace it with an indirect complement placed just in front of this verb. "J'aime le chocolat" => "je l' aime". So, "J'aime toi" (which is a form that no one will ever used because it sounds ugly) becomes "Je t' aime".
The simple answer is "because it is the rule".
The rule says that object pronouns are always placed before the verb except in imperative affirmative sentences.
J'aime la France. → Je l'aime.
J'entends les oiseaux. → Je les entends.
An easy lesson on the subject on Bonjour de France.