Science is one of those nouns that can be both countable and uncountable, but it is generally uncountable. We say things like ‘Science has made great progress over the last 100 years’ or ‘More men than women study science’. When it is countable, we usually have something more specific in mind. The sciences means physics, chemistry, biology and a few others. We can also speak of the natural sciences and the social sciences. You’re right to find that I love sciences sounds wrong. The plural, I would guess, rarely, if at all, occurs without the definite article.
In current English, humanities, generally means, in the Oxford English Dictionary’s definition:
The branch of learning concerned with human culture; the academic subjects collectively comprising this branch of learning, as history, literature, ancient and modern languages, law, philosophy, art, and music.
As such, it almost always occurs in the plural. However, the OED goes on to say
Hence also in singular: any one of these subjects.
In support of this use there is this citation from 1991:
Art history is a humanity and should be taught as such.
All the OED’s citations that use the word in the plural are preceded by the. It is, however, just about conceivable that it could occur without.