During the wedding scene in GF1, Tom Hagen is going over some business with the Don. When Tom mentions that Luca Brassi wants to see him,the Don replies "Is this really necessary?". It sounded like he didn't want to meet with Luca. What would be the reason for that? Wasn't this guy a close friend?

I dont think it was for any other reason then, it was his daughters wedding and he wanted to spend time with her when one thing popped up after another.

Luca made Vito nervous for several reasons:
First, the novel describes Luca as "...a natural force...like lightning..." He was so fierce, so violent, so un-subject to the usual constraints of behavior, that even Vito, who commanded his loyalty, could not be totally certain that Luca wouldn't do something crazy or at least embarrassing.
Second, Vito wanted Connie's wedding to be a respectable affair. Luca's fearsome appearance (and ever-present violence, which Puzo describes in the novel as "...a violet fury...", could detract from the dignity of the affair. Recall that Kay asks Michael, "Who's that scary-looking guy?" If she noticed, so did others.
Finally: Given Luca's fierce appearance (and probable prominence in law enforcement suspicions), Vito probably feared that being seen in close proximity to Luca might have provided fodder for law enforcement observers. Yes, we see that the "goddam FBI" is outside the compound's gates. But Vito, a (generally) careful guy, might have feared that some law enforcement undercover guy might have infiltrated the wedding and noted his private meeting with Luca. Far fetched, but in Mob life, nothing is totally far fetched.

Too true, TB. In the novel, the Don tells Michael that men like Luca and Al Neri are violent, dangerous men. They don't have any fear, not even of death. Their violent tempers actually have them asking for death each and every day. Since they fear nothing, not even death, the only fear they have is that their Don will be the one to kill them.

The Don always made sure to treat Luca with respect and dignity, but I think that, on some level, Luca's violent nature disgusted the Don. He preferred not to think about that side of the business, but I believe he felt that Luca was necessary to carry out certain things. The Don didn't like violent anger. He preferred subtlety.

Good points TB, and to contrast Vito's hope the wedding would be "respectable" there is a great scene where Tom tells the somewhat disappointed Don that various judges and politicians did not come to the wedding, but that "they all sent gifts."