While a woman is nidda and she and her husband are observing harchakot, is it permitted for them to tell each other, "I love you," or is that too suggestive of a statement for the nidda period?
Someone asked this of Rabbi Yaakov Hopfer, a major posek on these matters in Baltimore. He said without hesitation that it was permissible.
His interpretation of the prohibition on "s'chok vekalut rosh" is "behavior that is suggestive or disinhibiting." I don't see a normal "I love you" as either of those.
A person whose wife is nidah is still obligated to love her as much as he loves himself; anything he says in order to "lessen the tension in the air" is permitted (Nit'ei Gavriel 33:4 and footnote 8). So I guess to say "I love you" to "lessen the tension in the air" is permitted, but to say it for no reason may be closer to lightheadedness.