The old saying "opposites attract" suggests which reason for forming relationships?

The answer is complementary since both have the same feelings that can be developed as a relationship.  Although there were differences in both persons, they might see each other's characteristics as helpful to them. Their attributes can help each other develop the things that they lack and lead to improvement. 

productive most commonly means: achieving or producing a significant amount or result.

explanation:

productive can also mean:

producing or giving rise torelating to or engaged in the production of goods, crops, or other commodities.

these depend on how the word productive is used. most likely productive will mean   achieving or producing a significant amount or result.