It is very different in Japanese and in English.
Japanese use formal language in situations that English understand as casual.

But frankly, I do not hear (in movies) aishteru at all.
Only in songs. Maybe I will be corrected, but they usually use suki (read ski) or daisuki.
Girls usually that way or with desu, boys usually with da. So sukida for a boy is pretty common.

Concerning personal pronoun, they are not usually used in Japanese.
Watashi (me) is really obvious in that situation and can be used only for emphasis usually by girls.
For boys, it is very formal. If he uses it, he would say boku.
Anata (you) is used in Japanese rarely so it often signs of passion and it is used in that situation usually by girls, commonly in anatanokoto form.
In Japanese is common addressing by name, so it is not rare to say for example Yuta no koto ga daisuki desu (Yuta is a name of the forthcoming boyfriend, and desu is here, so well, it is natural for girls to speak more "formally").

And for understanding, it is love confession, not every day's way to display affection.