On a valentine’s day during dinner with a girl and the lovely view ?
There is no ambiguity about it, it’s definitely in all seriousness ! 🙂

Well it really means “I love you…”.

Many people nowadays throw ‘love’ around like it doesn’t have any true meaning, so the real answer is whatever loving someone means to you. Many people have different interpretations of love.

It is definitely more serious than:

Je t’aime bien.

That just means “I like you”, not to a significant other, but as a friend, pal, buddy, etc.

The bottom line is however you interpret ‘love’ is what “Je t’aime” means.

In France, it’s “serious”.

Sometime a Mom could also say that to her child or the reverse, though (but I suspect it’s an Americanism, and not that frequent).

On peut toujours dire « je t’aime » à une fille, elle en fait ce qu’elle veut !
Je t’aime peut se traduire par : ”I love you” ou bien ”I like you” !