How did Romeo and Juliet's love for each other lead them to isolation?
Often young lovers will become so focused on one another that they forget about the others. In the beginning of the story, Shakespeare seems to take care to establish how close Romeo is to Benvolio and Mercutio. Benvolio comforts Romeo for losing Roasline in the first scene.
Alas that love, so gentle in his view,
Should be so tyrannous and rough in proof! (Act 1, Scene 1, enotes etext pdf p. 15)
These friends may tease him, but they clearly do care about him. Yet Romeo certainly does not tell them what is going on. If he had, they might have been more careful and it might even have saved Mercutio. His comment to Romeo can be interpreted both ways.
Why the devil came you between
us? I was hurt under your arm. (Act 3, Scene 1, p. 63)
By contrast, Romeo seems to keep ties with Friar Lawrence. This is because Friar Lawrence can strengthen his ties to Juliet by marrying them. The friar seems to approve of the relationship in terms of what it can accomplish, by ending the feud.
O, she knew well(90)
Thy love did read by rote, and could not spell.
But come, young waverer, come go with me.
In one respect I'll thy assistant be;
For this alliance may so happy prove
To turn your households’ rancour to pure love.(95) (Act 1, Scene 3, p. 47)
Juliet does not really seem to have strong familial relationships. Her parents are distant and domineering. Yet Juliet’s relationship with her nurse is strong until she marries Romeo. Then she begins to move away from Nurse and keep secrets. Romeo is all she knows now. Juliet lies to Nurse and tells her she is going to confess, and that is why she visits Friar Lawrence.
Well, thou hast comforted me marvellous much.
Go in; and tell my lady I am gone,
Having displeas'd my father, to Laurence’ cell,
To make confession and to be absolv'd. (Act 3, Scene 5, p. 85)
Perhaps if everyone had not tried to force her to marry Paris, and had actually asked her what she wanted, Juliet would have remained close to someone else and things would have ended up differently.