Romantic relationships are the most difficult, but other kinds of relationships have their fair share of stress also. The three big factors the lack of which can stress a relationship are tolerance, communication and honesty.
If people enter any kind of relationship with fixed stereotypes and unrealistic expectations, lack of tolerance will stress the relationship right from the start. Being judgmental and expressing disdain definitely does not help.
Communication problems stress relationships whether they are intentional or unintentional. Words are inherently inaccurate in conveying thoughts and emotions, and any such small error is quickly compounded if communication is not held in high esteem by all involved parties.
Last but not least: honesty. If you are not sincere with yourself, you have a recipe for stress of disastrous proportions right here. When a relationship is burdened with shady motives and trust issues - don't expect to forge a strong bond.
The secret of reducing relationship stress to a minimum is to be sincere with yourself first and others second, have hopes but no expectations, and do your best to express yourself clearly.