I've played Benedick before, but it wasn't until I saw a video of a Canadian production that I realized the meaning behind the following exchange:
Don Pedro: I believe this is your daughter?
Lenoato: Her mother hath many times told me so.
Benedick: Were you in doubt sir, that you asked her?
Leonato: No, Signor Benedick, for then you were a child.
Yes, Benedick is implying that Leonato was a cuckold and that his daughter Hero was born illegitimately, but he's also implying that he himself is the father. Leonato brushes him off by saying that he was too young to have dealt in those Country Matters. -A Troper Who Legitimately Hasn't Signed Up
It is subtle and just mentioned in passing, but it is suggested at one point that Beatrice and Benedick loved each other in the past, before something happened to them. What an interesting back story that would be...
Don Pedro: Come, lady, come; you have lost the heart of Signior Benedick.
Beatrice: Indeed, my lord, he lent it me awhile; and I gave him use for it, a double heart for his single one: marry, once before he won it of me with false dice, therefore your grace may well say I have lost it.
The "something that happened" was likely Beatrice wanting to get married and Benedick wishing to remain unwed.
The 2012 Joss Whedon version makes this point explicit: flashbacks show Benedick and Beatrice were lovers in the past.
Moreover, there are overtones that the double heart she gave him, and that which he won of her with "false dice" is her maidenhead. He got her to sleep with him, then dumped her. She's facing social ostracism as Hero did if the truth ever comes out, which suggests another reason why she doesn't seek to marry.