Security

Before I get into this one I need to stress that (in my humbled opinion) there is nothing wrong with this combination. I believe (and to name a figure at random) that at least 90% of all couples fall into this category. 

It's the situation where either 2 self sufficient people come together or when 2 people who's shapes match the other person's gap nicely, come together. Although the colours of the shapes do not match perfectly, the shapes fit well (and this is rare enough) so it's well worth a go in a relationship sense. 

The issues that are present (and commonly seen if you look around or have been there yourself) are that the shapes aren't actually perfect matches, so there's always that ever so slight bit of doubt. Your partner may be super sexy, have great dress sense, be an awesome cook and have an amazing job, but their snoring is immense and keeps you awake as well as irritates you to doom! Such a small and basic thing but enough to make a shape imperfect. What if the individual met another person who had all the positives and also didn't snore?  (I know you'll wanna argue this so we'll come back to it).

There's also the element of change. If you told your partner you couldn't stand the snoring and that they'd have to do something about it, all they'll hear is "my partner doesn't like me the way I am" or "I'd better sort this out or they might leave me". This can go a number of ways but most would result in the shape and or the colour changing for one of both people, eventually making them slowly drift apart through resentment or unrewarded efforts.

Back to the argument. Lets imagine our couple as they are. Now lets pop another woman (or man.. lets keep his options open) on his left. Regardless of whether his shape fits their space, lets assume that their shape fits his space perfectly (as does his current partners), but lets also imagine that it's pink (and so would appear to be a perfect match for him). In this situation it's clear that his attention would leave his partners and turn to this new 'intruder'. Just like in the first Compatibility image.

The argument is for those who say that this couple may have been together for 20 years and have so much history so he'd never leave her ever! This is when it comes down to the individual. This is clearly why people cheat, have affairs or act malicious in general. In most cases that seemingly perfect match was only going to be short lived and the effects of any actions taken are almost guaranteed to change the compatibility of the original couple. 

Personally I feel that it's not worth it, and that couples in this category are much better off if they both accept each other as they are, for their strengths, and weaknesses, and allow their companionship/love to escalate. Though i'm confident that most others may feel differently.