Porque hay verbos que siempre van seguidos del verbo en gerundio. Enjoy es uno de esos, junto a avoid, dislike, mind, finish. Para mí, la cuestión es de memorizar cuales van seguidos de infinitivo y cuales de gerundio.

You enjoy a thing. If that thing is an action, then you need to put it in gerund form to show that it works as a noun. If you don't do this, it still looks like a verb and your sentence turns out wrong. Some verbs are followed by gerunds. You can look this up quite easily. In the case where a word could work as a verb or a noun, you add a determiner (eg. an article or possessive): "I enjoyed the talk with her", or "I enjoyed my talk with her." Pay attention to prepositions - you can't use "to" in those examples. However, using "the talk" instead of "talking" makes it sound as if you were discussing a serious matter. Perhaps one of you was in trouble? :)

"I enjoyed talking to her so much" "Talking" is the gerund form of "to talk". Some verbs are followed by the GERUND and some verbs are followed by the INFINITIVE form. As a general rule, if an action occurs either before or at the same time as the main verb, you use the gerund form. Ej: I enjoy talking. He does not like hiking. She practices swimming. If an action occurs after the main verb, you generally use the infinitive form after it. Ej: I want to talk now. I decided to go to the movies. She hurried to meet her friend. He wants to visit his girlfriend. That said, there are many lists of verbs which take the gerund or the infinitive, and you will probably learn them just through practice. This website has a fairly good list: http://www.myenglishpages.com/site_php_files/grammar-lesson-gerund-infinitive.php#.VEV5CfnF-So