Hi M,

There are a number of subjective thoughts or sayings that can trigger the subjunctive mood in French grammar.

For example,

Wish, emotion, doubt, desire, interrogation, negation (penser, croire) general statements.

The sentence that you are querying "J'aime que tu sois si attentif" expresses emotion as I see it and therefore the subjunctive mood.

However, you may find this link helpful going forward:-

https://www.lawlessfrench.com/subjunctivisor/aimer/

Here you can check for a given verb structure whether or not the subjunctive is required.

Bonne continuation.

Jim

It doesn't have to be as strong as a wish. It's a marker of subjectivity. I would contrast 'J'aime que tu sois si attentif' (which reflects my subjective wish that you be so attentive) with 'c'est vrai que tu es si attentif' (which reflect the objective fact that you are so attentive).

To add to Jim’s response, ultimately there are many French expressions that simply require the subjunctive. For many, a reason can be deduced, for some it is not obvious. Unless you have a specific interest or need to explain the grammar, it is likely to be less  frustrating in the long term to just acquire these expressions as you come across them and accept this is the case. 

Because of the ‘subjectivity’, and the role of ‘doubt’, in using the subjunctive, as Tom refers to above, for those expressions that do not always need the subjunctive, it can be required sometimes. An example “ il est/c’est vrai que … “ in interrogative or negative statements.  

Apart from the excellent link Jim provided, you may find the link below useful for developing a ‘feel’ for the subjunctive. 

https://www.frenchtoday.com/blog/french-verb-conjugation/understanding-french-subjunctive/