I want to start out by saying these are probably the couples that will happen but these are the reason why I don't ship them as much as some others. I'm sorry if what I write seems like bashing but in all honesty I am never probably going to be a novelist like Lucy. So I'm sorry Gruvia, Nalu, Gale and Jerza fans if you want to turn away now please do, unless you want to try to change my opinion which I'm more then open too.
The reason I don't ship most the canon couples as much as others is because I don't seem them as the most healthy relationships.
Gruvia: Let me start by saying this is probably the one I'm most bias on, and personally dislike the most. However, it's probably not for the reason you think. When the manga and anime started I loved both Juvia and Gray characters. Gray was a quiet badass, who seemed like he was a bit of a flirt which I enjoyed. Juvia was an awesome mage who had the coolest powers in my mind. I loved how she was another extremely strong female (I believe it goes Erza, Mira and then Juvia in Fairy Tail for strongest female mage).
Now I get she had a very difficult past and in a way Gray saved her from it, he literally showed her the light. I feel like at first though she was focused on his looks and feel for partially because of his looks. To me relationships should be based of personality. However, she didn't know Gray's personality at first. Some could say it was love at first sight which I could see, so that's a valid point.
However, she also became very obsessed with Gray which I see as unhealthy. I didn't like how in the Grand Magic Games she wished for a team mate to get hurt so she could fight by Gray side, since that wasn't the Juvia I fell in love with I thought. The Juvia I loved was a bit different sure, that's to be expected after not having many true friends before Fairy Tail. She was strong and above all she valid the friendships she had at Fairy Tail. I also didn't personally like how she saw their friends as love rivals, since it seemed like she was threatened by Gray having other female friends. A healthy relationship to me is based on trust. Now I am not currently up to date with the manga so I hope a few things have changed in the one year time skip, because I think if Juvia had more self cofidence, this couple could work well (it's ice and water after all). However, I understand the lack of self confidence is part of her character but I think she's come so far so I would love to see her continue to grow.
Nalu: Natsu along with Happy (even though more happy) have both made fun of Lucy weight. I dislike this due to there being many woman who worry about there weight daily. I personally think Natsu being one of Lucy's best friend should ask Happy to stop this behavior along with him also stopping it because it hurts Lucy. I also didn't personally like how Natsu just left Lucy during the one year time skip, they're partners. He didn't even write, I think he should of thought of her feelings more during this period of time, because it truely hurt her.
I do find Natsu and Lucy very cute together at times, however I think due to me also shipping Nali and Graylu it makes it more difficult for me to ship these two completely. Even though I realize they'll most likely happen but I also realized I rarely ship the two main characters together and I don't know why. Maybe it's because I love it when the "sick kick" gets the girl.
Gale: I want to first say I've come to love Gajeel and while I do find Gale scene cute I can't fully support them because Gajeel attacked her. In the real world I would never want someone to get with someone who put them through that. I do think Gajeel is trying to make up for his past through, so this couple has me at an inner war with myself.
Jerza: Jellal put Erza through so much but he was being controlled so this one I can forgive me. He had her friends turn against her and fought her, however in the end he also saved her life. So this is the one that I probably support the most out of the semi-cannon couples.
Please remember these are just my opinions, I think everyone free to ship who they want. To some Gruvia maybe the perfect couple and that's wonderful, it's just not my idea of love.