Haven't read through all the answers, but it definetly should have a happy ending.
Of course not for all love interests. Only for the one you romanced in ME1 and only if you not "cheated" on him/her in ME2 (or rejoined with Liara in LOTSB). Furthermore you have to have a high enough War Asset (otherwise the LI is toast) AND you have to stay true to your cause (meaning paragon or renegade, but true to it through all 3 parts).
Furthermore I'd like answers... or conclusions. Like in Baldurs Gate 2 TOB, where you got some text to each character who survived, explaining his/her future depending on your decisions and relationship. This would be grat for all the team members that survived ME 1, 2 and 3, as well what happens to the galaxy now - also depending on your choices.
And last but not least: A post credit and post-everything family scene if you successfully got the full romance with Ashley/Kaidan/Liara, showing you and some (blue) children a few years after that, where your LI finishes telling the story of Shepards and his/her adventures, then one of the kids asks the same as in the post-credit stargazer scene, and Shepard answers "Soon, my dear".
Only thinking about that ending, makes me somewhat happy, now that I can imagine it