Intertwining tree roots.
When you start to share a house with someone and your possessions get mixed together and you begin to share each other's friends and activities, this is an example of a time when your lives begin to intertwine.
tr. & intr.v.in·ter·twined, in·ter·twin·ing, in·ter·twines
(third-person singular simple present intertwines, present participle intertwining, simple past and past participle intertwined)
inter- + twine