You'll also notice that whenever asked about him, Donald Trump is saying nice stuff about Ted Cruz.
Some things to consider about this...
Trump is a New Yorker. He needs a Texas running mate for the same reason Kennedy needed Johnson. Not only to help him in a potential general election, but to also help him with "conservatives" in the nomination process.
Cruz is hispanic. Trump needs a hispanic running mate to win back the hispanic vote.
Cruz is tea party. Trump wants that vote too.
Cruz is anti-establishment. Same as Trump.
And lastly but not leastly, Cruz has now fully adopted the Trump campaigning style. He's called the Senate Majority Leader in his own political party a "liar".