Tip #306: The romantic climax

the implied statement {of a proposal}: I want to spend the rest of my life with you, forsaking all others, etc, from someone you feel the same way about - that is where the emotion comes in. If you want to up that emotion, a really romantic proposal works to an extent, but you get the most impact from... the reader {and character(s) themselves being} unable to see a way for these two characters they really care about to live happily ever after, so when it happens, it blows them away

...if {the readers are} in secret of the planning, then they get to anticipate and fear the proposal scene

http://www.nanowrimo.org/en/forums/plot-doctoring/threads/101328 Comment by inkpawprints.

Tip #238: Extra spice

Adding a love interest is an easy way to add a bit of extra suspense, drama & conflict, to darken the dark point and sweeten the resolution of the main plot.

http://critiquecircle.com/forums.asp?action=viewforum&index=248&thread=950095

Tip #225: Not just a prize

The role of the romantic interest is to challenge the protagonist and shock her out of her regular mental and emotional pattern.

http://screenwritingtips.blcklst.com/2012/02/screenwriting-tip-904/

Tip #140: The "love interest"

there’s a profound difference between a love interest whose only investment in the plot is their attachment to the hero, and a fully functioning character who develops into a love interest.

http://fozmeadows.wordpress.com/2011/06/27/bechdel-matters/

Tip #69: Men actually like romance...

Men actually like romance...To be specific, men like the dynamic.  Men like seeing how it matures and how it evolves and how they eventually get together...  It’s a story, same as seeing how the hero overcomes the forces of evil.

http://samsykes.com/2011/01/sam-sykes-touches-men-and-women/
{Paraphrased:} and a story where one or both of the characters are passive isn't terribly interesting.