When people are unhappy with their own lives (and most people are), they don’t always admit this unhappiness to themselves. Instead, they will transfer the responsibility for their feelings to those around them, making those people at fault for how they feel. This will come up in a number of ways, but most notably, it makes already present resentments worse. If they would feel mildly annoyed by something, that minor irritation becomes anger. If they are already resentful and envious of certain aspects of your relationship, they may feel a need to actively do something about it, to try to poison it.
If they have an emotional connection to one or both of you then this will give them the cover they need to do it. They can offer advice and claim to do it out of love or concern. They can interfere and say that they are doing it for your own good. It may also give them a certain amount of power in your relationship. This is where mothers-in-law get their bad reputations.
That power is another reason they may do it, entirely separate from their own personal unhappiness. Power is addictive, and the fact that somebody is able to manipulate you or your partner, means that it will be at least tempting for them to do it.
Ways to defend against this:
Understand that not everybody who offers advice has your best interest in mind. Learn to see the possible ulterior motives when people appear eager to help.
Do not seek help when your are emotional. Granted, that’s when you most want it, but doing this removes much of your objectivity, and makes you overly receptive to manipulative suggestions.
Settle problems within the relationship. If it’s necessary to bring somebody in, bring in somebody wholly unconnected with any of the parties.